leetcode解题: Merge Two Sorted List (21)

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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.

解法1：Dummy Node O(N + M)

对于head不明确的linkedlist的题目，都考虑建立Dummy Node，然后返回Dummy->next的办法解决。
C++
用C++注意在最后需要delete掉dummy防止memory leak

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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) {
        ListNode* dummy = new ListNode(0);
        ListNode* tail = dummy;
        while (l1 != NULL && l2 != NULL) {
            if (l1->val <= l2->val) {
                tail->next = l1;
                l1 = l1->next;
            } else {
                tail->next = l2;
                l2 = l2->next;
            }
            tail = tail->next;
        }
        while (l1 != NULL) {
            tail->next = l1;
            l1 = l1->next;
            tail = tail->next;
        }
        while (l2 != NULL) {
            tail->next = l2;
            l2 = l2->next;
            tail = tail->next;
        }
        ListNode* res = dummy->next;
        delete dummy;
        return res;
    }
};

Java

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