[leetcode]Merge Two Sorted Lists

Merge Two Sorted Lists

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.

算法思路：

以l1为基准，拿l2的点逐个插入到l1中，两个链表都是一次遍历，我这里没有做优化，时间复杂度O(m+n)，空间复杂度O(1)

 

public class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if(l1 == null) return l2;
        if(l2 == null) return l1;
        ListNode head = new ListNode(1);
        head.next = l1;
        ListNode pre = head;
        ListNode node = l2;
        while(l2 != null){
            node = l2;
            while(pre.next != null && pre.next.val <= node.val){
                pre = pre.next;
            }
            l2 = node.next;
            node.next = pre.next;
            pre.next = node;
            pre = node;//如果这句删掉的话，相当于l2扫描了两边，l1扫描一遍，但是时间复杂度不变
        }
        return head.next;
    }
}