Merge 2 sorted lists@leetcode

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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.

» Solve this problem

Different from merge k sorted lists, this problem asks for merge in place. So we can't create a new list to put the merged lists. So here, I opened two pointers which point to two different lists, and compare. The idea is exactly the same as merge sort.

When using the pointers to make change, we need to be very careful that we are changing the right one!

There is another version that using multiset to do this problem which is more concise, while is more expensive.

Multiset is a set whose elements obey the weak ordering rule. Multiset is slower than unordered_multiset. And normally, the unordered_multiset is implemented by binary search tree.

  
/**
 * 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||!l2) return l1==NULL? l2 : l1;
        ListNode *pre = new ListNode(-1);
        ListNode *cur = pre;
        while(l1&&l2){
            if(l1->val>l2->val){
                cur->next = l2;
                cur = cur->next;
                l2 = l2->next;
        }
            else{
                cur->next = l1;
                //if(!pre->next) pre->next = cur;
                cur = cur->next;
                l1 = l1->next;
            }
        }
        if(l1||l2)
        cur->next = l1==NULL? l2 : l1;

        ListNode *head = pre->next;
        delete pre;
        return head;
    }
    };