【LeetCode】21.合并两个有序链表

/**
 * 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) {
        ListNode *l;
        ListNode *temp;
        ListNode *ans;
        if(l1==NULL) return l2;
        if(l2==NULL) return l1;
        if(l1->val>l2->val){
            l=l2;
            l2=l1;
            l1=l;
        }
        ans=l1;
        while(l2!=NULL){
            if(l1==NULL) return ans;
            if(l1->next==NULL||l2->val<l1->next->val){
                l=l1->next;
                temp=l2->next;
                l1->next=l2;
                l2->next=l;
                l1=l2;
                l2=temp;
            }else{
                l1=l1->next;
            }
        }
        return ans;
    }
};