Today I have done leetcode's March LeetCoding Challenge with cpp.

March LeetCoding Challenge 20

Description

Merge In Between Linked Lists

You are given two linked lists: list1 and list2 of sizes n and m respectively.

Remove list1's nodes from the ath node to the bth node, and put list2 in their place.

The blue edges and nodes in the following figure indicate the result:

Build the result list and return its head.

Example 1:

Input: list1 = [10,1,13,6,9,5], a = 3, b = 4, list2 = [1000000,1000001,1000002]
Output: [10,1,13,1000000,1000001,1000002,5]
Explanation: We remove the nodes 3 and 4 and put the entire list2 in their place. The blue edges and nodes in the above figure indicate the result.

Example 2:

Input: list1 = [0,1,2,3,4,5,6], a = 2, b = 5, list2 = [1000000,1000001,1000002,1000003,1000004]
Output: [0,1,1000000,1000001,1000002,1000003,1000004,6]
Explanation: The blue edges and nodes in the above figure indicate the result.

Constraints:

• 3 <= list1.length <= 104
• 1 <= a <= b < list1.length - 1
• 1 <= list2.length <= 104

Solution

class Solution {
public:
  ListNode* mergeInBetween(ListNode* list1, int a, int b, ListNode* list2) {
    ListNode *head = list1;
    while(--a && --b) {
      list1 = list1->next;
    }
    ListNode *rest = list1->next;
    list1->next = list2;
    
    while(list1->next) list1 = list1->next;
    
    while(b--) rest = rest->next;
    list1->next = rest;
    
    return head;
  }
};