leetcode 21. Merge Two Sorted Lists

Posted 2021-02-07 jamieliu

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.

Example:

Input: 1->2->4, 1->3->4
Output: 1->1->2->3->4->4

 

庆祝第一次bug free！！(虽然不是最快的解法）

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode head = new ListNode(0);
        ListNode tail = head;
        int i=0, j=0;
        while(l1!=null || l2!=null) {
            if(l1 != null && l2 != null) {
                if(l1.val <= l2.val) {
                    tail.next = l1;
                    tail = tail.next;
                    l1 = l1.next;
                }else{
                    tail.next = l2;
                    tail = tail.next;
                    l2 = l2.next;
                } 
            }else if(l1 == null){
                tail.next = l2;
                tail = tail.next;
                l2 = l2.next;
            }else{
                tail.next = l1;
                tail = tail.next;
                l1 = l1.next;
            }
            
        }
        return head.next;
    }
}

 

解法二：递归

好不容易看懂.......看来只能把答案背下来了 （faster than 100%.......）

public ListNode mergeTwoLists(ListNode l1, ListNode l2){
        if(l1 == null) return l2;
        if(l2 == null) return l1;
        if(l1.val < l2.val){
            l1.next = mergeTwoLists(l1.next, l2);
            return l1;
        } else{
            l2.next = mergeTwoLists(l1, l2.next);
            return l2;
        }
}