Merge two Sorted Lists

Intuition

Extract all elements from both linked lists into an additional array, sort the array, and then create a new combined linked list from the sorted elements.

Approach

Initialize an empty array and iterate through each node of both the first and second linked lists, adding their elements to the array. This ensures that all elements from both lists are collected in one place for easy sorting.
Sort the array in ascending order using a sorting algorithm such as quick sort, merge sort, or an inbuilt library function. Sorting the array ensures that the combined elements from both lists are in the correct order for merging.
Create a new linked list from the sorted array. Initialize a dummy node to act as the head of the merged linked list. Iterate through the sorted array, creating a new linked list node for each element, and link these nodes together. Finally, return the next node of the dummy node as the head of the merged sorted linked list.

Dry Run

Solution

#include <bits/stdc++.h>
using namespace std;

// Definition of singly linked list
struct ListNode {
    int val;
    ListNode *next;
    ListNode(int data1) {
        val = data1;
        next = NULL;
    }
    ListNode(int data1, ListNode *next1) {
        val = data1;
        next = next1;
    }
};

class Solution {
public:
    // Function to merge two sorted linked lists
    ListNode* mergeTwoLists(ListNode* list1, ListNode* list2) {
        vector<int> arr;
        ListNode* temp1 = list1;
        ListNode* temp2 = list2;

        
        // Add elements from list1 to the vector
        while(temp1 != NULL){
            arr.push_back(temp1->val); 
            // Move to the next node in list1
            temp1 = temp1->next; 
        }

        // Add elements from list2 to the vector
        while(temp2 != NULL){
            arr.push_back(temp2->val);
            // Move to the next node in list2
            temp2 = temp2->next; 
        }

        // Sorting the vector in ascending order
        sort(arr.begin(), arr.end());

        // Sorted vector to linked list
        ListNode* dummyNode = new ListNode(-1);
        ListNode* temp = dummyNode;
        for(int i = 0; i < arr.size(); i++){
            temp->next = new ListNode(arr[i]); 
            temp = temp->next; 
        }

        // Return the head of 
        // merged sorted linked list
        return dummyNode->next; 
    }
};

// Function to print the linked list
void printLinkedList(ListNode* head) {
    ListNode* temp = head;
    while (temp != nullptr) {
        // Print the data of the current node
        cout << temp->val << " "; 
        // Move to the next node
        temp = temp->next; 
    }
    cout << endl;
}

int main() {
    // Example Linked Lists
    ListNode* list1 = new ListNode(1);
    list1->next = new ListNode(3);
    list1->next->next = new ListNode(5);

    ListNode* list2 = new ListNode(2);
    list2->next = new ListNode(4);
    list2->next->next = new ListNode(6);

    cout << "First sorted linked list: ";
    printLinkedList(list1);

    cout << "Second sorted linked list: ";
    printLinkedList(list2);

    Solution solution;
    ListNode* mergedList = solution.mergeTwoLists(list1, list2);

    cout << "Merged sorted linked list: ";
    printLinkedList(mergedList);

    return 0;
}
import java.util.*;

// Definition of singly linked list
class ListNode {
    int val;
    ListNode next;
    ListNode(int data1) {
        val = data1;
        next = null;
    }
    ListNode(int data1, ListNode next1) {
        val = data1;
        next = next1;
    }
}

class Solution {
    // Function to merge two sorted linked lists
    public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
        ArrayList<Integer> arr = new ArrayList<>();
        ListNode temp1 = list1;
        ListNode temp2 = list2;

        
        // Add elements from list1 to the vector
        while (temp1 != null) {
            arr.add(temp1.val);
            // Move to the next node in list1
            temp1 = temp1.next;
        }

        // Add elements from list2 to the vector
        while (temp2 != null) {
            arr.add(temp2.val);
            // Move to the next node in list2
            temp2 = temp2.next;
        }

        // Sorting the vector in ascending order
        Collections.sort(arr);

        // Sorted vector to linked list
        ListNode dummyNode = new ListNode(-1);
        ListNode temp = dummyNode;
        for (int i = 0; i < arr.size(); i++) {
            temp.next = new ListNode(arr.get(i));
            temp = temp.next;
        }

        // Return the head of 
        // merged sorted linked list
        return dummyNode.next;
    }
}

// Function to print the linked list
public static void printLinkedList(ListNode head) {
    ListNode temp = head;
    while (temp != null) {
        // Print the data of the current node
        System.out.print(temp.val + " ");
        // Move to the next node
        temp = temp.next;
    }
    System.out.println();
}

public static void main(String[] args) {
    // Example Linked Lists
    ListNode list1 = new ListNode(1);
    list1.next = new ListNode(3);
    list1.next.next = new ListNode(5);

    ListNode list2 = new ListNode(2);
    list2.next = new ListNode(4);
    list2.next.next = new ListNode(6);

    System.out.print("First sorted linked list: ");
    printLinkedList(list1);

    System.out.print("Second sorted linked list: ");
    printLinkedList(list2);

    Solution solution = new Solution();
    ListNode mergedList = solution.mergeTwoLists(list1, list2);

    System.out.print("Merged sorted linked list: ");
    printLinkedList(mergedList);
}
# Definition of singly linked list
class ListNode:
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next

class Solution:
    # Function to merge two sorted linked lists
    def mergeTwoLists(self, list1: ListNode, list2: ListNode) -> ListNode:
        arr = []
        temp1 = list1
        temp2 = list2

        
        # Add elements from list1 to the vector
        while temp1:
            arr.append(temp1.val)
            # Move to the next node in list1
            temp1 = temp1.next

        # Add elements from list2 to the vector
        while temp2:
            arr.append(temp2.val)
            # Move to the next node in list2
            temp2 = temp2.next

        # Sorting the vector in ascending order
        arr.sort()

        # Sorted vector to linked list
        dummyNode = ListNode(-1)
        temp = dummyNode
        for val in arr:
            temp.next = ListNode(val)
            temp = temp.next

        # Return the head of 
        # merged sorted linked list
        return dummyNode.next

# Function to print the linked list
def printLinkedList(head: ListNode):
    temp = head
    while temp:
        # Print the data of the current node
        print(temp.val, end=" ")
        # Move to the next node
        temp = temp.next
    print()

if __name__ == "__main__":
    # Example Linked Lists
    list1 = ListNode(1)
    list1.next = ListNode(3)
    list1.next.next = ListNode(5)

    list2 = ListNode(2)
    list2.next = ListNode(4)
    list2.next.next = ListNode(6)

    print("First sorted linked list: ", end="")
    printLinkedList(list1)

    print("Second sorted linked list: ", end="")
    printLinkedList(list2)

    solution = Solution()
    mergedList = solution.mergeTwoLists(list1, list2)

    print("Merged sorted linked list: ", end="")
    printLinkedList(mergedList)
// Definition of singly linked list
class ListNode {
    constructor(val = 0, next = null) {
        this.val = val;
        this.next = next;
    }
}

class Solution {
    // Function to merge two sorted linked lists
    mergeTwoLists(list1, list2) {
        let arr = [];
        let temp1 = list1;
        let temp2 = list2;

        
        // Add elements from list1 to the vector
        while (temp1) {
            arr.push(temp1.val);
            // Move to the next node in list1
            temp1 = temp1.next;
        }

        // Add elements from list2 to the vector
        while (temp2) {
            arr.push(temp2.val);
            // Move to the next node in list2
            temp2 = temp2.next;
        }

        // Sorting the vector in ascending order
        arr.sort((a, b) => a - b);

        // Sorted vector to linked list
        let dummyNode = new ListNode(-1);
        let temp = dummyNode;
        for (let i = 0; i < arr.length; i++) {
            temp.next = new ListNode(arr[i]);
            temp = temp.next;
        }

        // Return the head of 
        // merged sorted linked list
        return dummyNode.next;
    }
}

// Function to print the linked list
function printLinkedList(head) {
    let temp = head;
    while (temp) {
        // Print the data of the current node
        process.stdout.write(temp.val + " ");
        // Move to the next node
        temp = temp.next;
    }
    console.log();
}

let list1 = new ListNode(1);
list1.next = new ListNode(3);
list1.next.next = new ListNode(5);

let list2 = new ListNode(2);
list2.next = new ListNode(4);
list2.next.next = new ListNode(6);

console.log("First sorted linked list: ");
printLinkedList(list1);

console.log("Second sorted linked list: ");
printLinkedList(list2);

let solution = new Solution();
let mergedList = solution.mergeTwoLists(list1, list2);

console.log("Merged sorted linked list: ");
printLinkedList(mergedList);

Complexity Analysis

Time Complexity: O(N1 + N2) + O(N log N) + O(N) where N1 is the number of linked list nodes in the first list, N2 is the number of linked list nodes in the second list, and N is the total number of nodes (N1 + N2). Traversing both lists into the array takes O(N1 + N2), sorting the array takes O((N1 + N2) X log(N1 + N2)), and then traversing the sorted array and creating a list gives us another O(N1 + N2).

Space Complexity: O(N) + O(N) where N is the total number of nodes from both lists (N1 + N2). O(N) to store all the nodes of both the lists in an external array and another O(N) to create a new combined list.

Intuition

A more efficient approach involves merging the given sorted linked lists directly without the need for an intermediate array. By taking advantage of the fact that the linked lists are already sorted, we can use a simple comparison strategy. Position a pointer each at the beginning of both lists. Compare the current values at these pointers and choose the smaller value to add to the new merged list. Move the pointer forward in the list from which the smaller value was taken. Repeat this process until one of the lists is fully merged. At this point, attach the remaining nodes of the other list to the merged list, as they are already in order.

Approach

Set up two pointers at the start of each input linked list. Create a dummy node to serve as the anchor for the beginning of the merged list and use a separate traversal pointer to build the combined list.
While both lists have remaining nodes, compare their current values. Attach the smaller value to the merged list and advance the traversal pointer, along with the pointer from the list from which the smaller value was taken.
When one of the lists is fully traversed, directly attach the remaining nodes from the other list to the merged list, as they are already in sorted order.
The merged list starts after the dummy node. Return the next node of the dummy node as the head of the newly merged and sorted linked list.

Dry Run

Solution

#include <bits/stdc++.h>

using namespace std;

// Definition of singly linked list
struct ListNode {
    int val;
    ListNode *next;
    ListNode(int data1) {
        val = data1;
        next = NULL;
    }
    ListNode(int data1, ListNode *next1) {
        val = data1;
        next = next1;
    }
};

class Solution {
public:
    // Function to merge two sorted linked lists
    ListNode* mergeTwoLists(ListNode* list1, ListNode* list2) {
        // Create a dummy node to serve as 
        // the head of the merged list
        ListNode* dummyNode = new ListNode(-1);
        ListNode* temp = dummyNode;

        // Traverse both lists simultaneously
        while (list1 != nullptr && list2 != nullptr) {
            /*Compare elements of both lists 
            and link the smaller node 
            to the merged list*/
            if (list1->val <= list2->val) {
                temp->next = list1;
                list1 = list1->next;
            } else {
                temp->next = list2;
                list2 = list2->next;
            }
            // Move the temporary pointer 
            // to the next node
            temp = temp->next;
        }

        /*If any list still 
        has remaining elements, 
        append them to the merged list*/
        if (list1 != nullptr) {
            temp->next = list1;
        } else {
            temp->next = list2;
        }

        // Return merged list 
        return dummyNode->next;
    }
};

// Function to print the linked list
void printLinkedList(ListNode* head) {
    ListNode* temp = head;
    while (temp != nullptr) {
        // Print the data of the current node
        cout << temp->val << " ";
        // Move to the next node
        temp = temp->next;
    }
    cout << endl;
}

int main() {
    // Example Linked Lists
    ListNode* list1 = new ListNode(1);
    list1->next = new ListNode(3);
    list1->next->next = new ListNode(5);

    ListNode* list2 = new ListNode(2);
    list2->next = new ListNode(4);
    list2->next->next = new ListNode(6);

    cout << "First sorted linked list: ";
    printLinkedList(list1);

    cout << "Second sorted linked list: ";
    printLinkedList(list2);

    Solution solution;
    ListNode* mergedList = solution.mergeTwoLists(list1, list2);

    cout << "Merged sorted linked list: ";
    printLinkedList(mergedList);

    return 0;
}
import java.util.*;

// Definition of singly linked list
class ListNode {
    int val;
    ListNode next;
    ListNode(int data1) {
        val = data1;
        next = null;
    }
    ListNode(int data1, ListNode next1) {
        val = data1;
        next = next1;
    }
}

class Solution {
    // Function to merge two sorted linked lists
    public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
        // Create a dummy node to serve as 
        // the head of the merged list
        ListNode dummyNode = new ListNode(-1);
        ListNode temp = dummyNode;

        // Traverse both lists simultaneously
        while (list1 != null && list2 != null) {
            /*Compare elements of both lists 
            and link the smaller node 
            to the merged list*/
            if (list1.val <= list2.val) {
                temp.next = list1;
                list1 = list1.next;
            } else {
                temp.next = list2;
                list2 = list2.next;
            }
            // Move the temporary pointer 
            // to the next node
            temp = temp.next;
        }

        /*If any list still 
        has remaining elements, 
        append them to the merged list*/
        if (list1 != null) {
            temp.next = list1;
        } else {
            temp.next = list2;
        }

        // Return merged list 
        return dummyNode.next;
    }
}

// Function to print the linked list
public static void printLinkedList(ListNode head) {
    ListNode temp = head;
    while (temp != null) {
        // Print the data of the current node
        System.out.print(temp.val + " ");
        // Move to the next node
        temp = temp.next;
    }
    System.out.println();
}

public static void main(String[] args) {
    // Example Linked Lists
    ListNode list1 = new ListNode(1);
    list1.next = new ListNode(3);
    list1.next.next = new ListNode(5);

    ListNode list2 = new ListNode(2);
    list2.next = new ListNode(4);
    list2.next.next = new ListNode(6);

    System.out.print("First sorted linked list: ");
    printLinkedList(list1);

    System.out.print("Second sorted linked list: ");
    printLinkedList(list2);

    Solution solution = new Solution();
    ListNode mergedList = solution.mergeTwoLists(list1, list2);

    System.out.print("Merged sorted linked list: ");
    printLinkedList(mergedList);
}
# Definition of singly linked list
class ListNode:
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next

class Solution:
    # Function to merge two sorted linked lists
    def mergeTwoLists(self, list1: ListNode, list2: ListNode) -> ListNode:
        # Create a dummy node to serve as 
        # the head of the merged list
        dummyNode = ListNode(-1)
        temp = dummyNode

        # Traverse both lists simultaneously
        while list1 is not None and list2 is not None:
            '''Compare elements of both lists 
            and link the smaller node 
            to the merged list'''
            if list1.val <= list2.val:
                temp.next = list1
                list1 = list1.next
            else:
                temp.next = list2
                list2 = list2.next
            # Move the temporary pointer 
            # to the next node
            temp = temp.next

        '''If any list still 
        has remaining elements, 
        append them to the merged list'''
        if list1 is not None:
            temp.next = list1
        else:
            temp.next = list2

        # Return merged list
        return dummyNode.next

# Function to print the linked list
def printLinkedList(head: ListNode):
    temp = head
    while temp is not None:
        # Print the data of the current node
        print(temp.val, end=" ")
        # Move to the next node
        temp = temp.next
    print()

if __name__ == "__main__":
    # Example Linked Lists
    list1 = ListNode(1)
    list1.next = ListNode(3)
    list1.next.next = ListNode(5)

    list2 = ListNode(2)
    list2.next = ListNode(4)
    list2.next.next = ListNode(6)

    print("First sorted linked list: ", end="")
    printLinkedList(list1)

    print("Second sorted linked list: ", end="")
    printLinkedList(list2)

    solution = Solution()
    mergedList = solution.mergeTwoLists(list1, list2)

    print("Merged sorted linked list: ", end="")
    printLinkedList(mergedList)
// Definition of singly linked list
class ListNode {
    constructor(val = 0, next = null) {
        this.val = val;
        this.next = next;
    }
}

class Solution {
    // Function to merge two sorted linked lists
    mergeTwoLists(list1, list2) {
        // Create a dummy node to serve as 
        // the head of the merged list
        let dummyNode = new ListNode(-1);
        let temp = dummyNode;

        // Traverse both lists simultaneously
        while (list1 !== null && list2 !== null) {
            /*Compare elements of both lists 
            and link the smaller node 
            to the merged list*/
            if (list1.val <= list2.val) {
                temp.next = list1;
                list1 = list1.next;
            } else {
                temp.next = list2;
                list2 = list2.next;
            }
            // Move the temporary pointer 
            // to the next node
            temp = temp.next;
        }

        /*If any list still 
        has remaining elements, 
        append them to the merged list*/
        if (list1 !== null) {
            temp.next = list1;
        } else {
            temp.next = list2;
        }

        // Return merged list
        return dummyNode.next;
    }
}

// Function to print the linked list
function printLinkedList(head) {
    let temp = head;
    while (temp !== null) {
        // Print the data of the current node
        process.stdout.write(temp.val + " ");
        // Move to the next node
        temp = temp.next;
    }
    console.log();
}

// Example Linked Lists
let list1 = new ListNode(1);
list1.next = new ListNode(3);
list1.next.next = new ListNode(5);

let list2 = new ListNode(2);
list2.next = new ListNode(4);
list2.next.next = new ListNode(6);

console.log("First sorted linked list: ");
printLinkedList(list1);

console.log("Second sorted linked list: ");
printLinkedList(list2);

let solution = new Solution();
let mergedList = solution.mergeTwoLists(list1, list2);

console.log("Merged sorted linked list: ");
printLinkedList(mergedList);

Complexity Analysis

Time Complexity: O(N1 + N2) because both lists are traversed in a single pass for merging without any additional loops or nested iterations. Here N1 is the number of nodes in the first linked list and N2 is the number of nodes in the second linked list.
Space Complexity: O(1) because no additional data structures or space is allocated for storing data, only a constant space for pointers to maintain for traversing the linked list.

Problem List

Examples:

Constraints

Hints

Company Tags

Editorial

Intuition

Approach

Dry Run

Solution

Complexity Analysis

Intuition

Approach

Dry Run

Solution

Complexity Analysis

Frequently Occurring Doubts

Interview Followup Questions

Notes

Code