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leetcode 021: Merge Two Sorted Lists

https://leetcode.com/problems/merge-two-sorted-lists/

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problem:

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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

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start code (c++)

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solution:

我的思路：

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/**
 * 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)
            return l2;
        if (l2 == NULL)
            return l1;
        
        ListNode* head;
        ListNode* tail;
        if (l1->val <= l2->val){
            head = l1;
            l1 = l1->next;
        }
        else{
            head = l2;
            l2 = l2->next;            
        }
        
        tail = head;
        
        while (l1 && l2){
            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 = l2;
        else
            tail->next = l1;
        return head;
    }
};

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