[Leetcode 21] 21 Merge Two Sorted Lists

Problem:

Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.

Analysis:

like merge sort.

Time complexity is O(m+n), Space complexity is O(m+n)

Code:

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if (l1 == null) return l2;
        if (l2 == null) return l1;
        
        ListNode head = null, tail=null;
        while (l1 != null && l2 != null) {
            if (l1.val < l2.val) {
                if (head == null) {
                    head = l1;
                } else {
                    tail.next = l1;
                }
                
                tail = l1;
                l1 = l1.next;
            } else {
                if (head == null) {
                    head = l2;
                } else {
                    tail.next = l2;
                }
                
                tail = l2;
                l2 = l2.next;
            }
        }
        
        while (l1 != null) {
            tail.next = l1;
            tail = l1;
            l1 = l1.next;
        }
        
        while (l2 != null) {
            tail.next = l2;
            tail = l2;
            l2 = l2.next;
        }
        
        return head;
    }
}

A highly optimized version:

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        // Start typing your Java solution below
        // DO NOT write main() function
        
        ListNode head = new ListNode(0), tail=head;
        while (l1 != null && l2 != null) {
            if (l1.val < l2.val) {     
                tail.next = l1;
                l1 = l1.next;
            } else {
                tail.next = l2;
                l2 = l2.next;
            }
            
            tail = tail.next;
        }
        
       if (l1 != null) tail.next = l1;
       if (l2 != null) tail.next = l2;
        
        return head.next;
    }
}

Attention: