【leetcode】21. Merge Two Sorted Lists 两个有序链表的归并有序

1. 题目

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.

2. 思路

两路遍历每次merge小的到新的链里。

3. 代码

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        if (l1 == NULL && l2 == NULL) return NULL;
        if (l1 == NULL) return l2;
        if (l2 == NULL) return l1;
        
        ListNode* head = NULL;
        if (l1->val < l2->val) {
            head = l1;
            l1 = l1->next;
        } else {
            head = l2;
            l2 = l2->next;
        }
        ListNode* p = head;
        while (l1 != NULL && l2 != NULL) {
            if (l1->val < l2->val) {
                p->next = l1;
                l1 = l1->next;
            } else {
                p->next = l2;
                l2 = l2->next;
            }
            p = p->next;
        }
        if (l1 != NULL) p->next = l1;
        if (l2 != NULL) p->next = l2;
        return head;
    }
};