Merge Two Sorted Lists.cpp

/*
21. Merge Two Sorted Lists

You are given the heads of two sorted linked lists list1 and list2.

Merge the two lists in a one sorted list. The list should be made by splicing together the nodes of the first two lists.

Return the head of the merged linked list.

 

Example 1:

Input: list1 = [1,2,4], list2 = [1,3,4]
Output: [1,1,2,3,4,4]

*/



class Solution {
public:
    ListNode* mergeTwoLists(ListNode* list1, ListNode* list2) {
        if(list1==NULL) return list2;
        if(list2==NULL) return list1;
        ListNode* ans = new ListNode(-1);
        ListNode* tail=ans;
        while(list1!=NULL && list2!=NULL){
            if(list1->val < list2->val){
                tail->next=list1;
                tail=list1;
                list1=list1->next;
            }
            else{
                tail->next=list2;
                tail=list2;
                list2=list2->next;
            }
        }
        if(list1==NULL) tail->next=list2;
        else tail->next=list1;
        return ans->next;
    }
};