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

October LeetCoding Challenge 1

Description

Check If Array Pairs Are Divisible by k

Given an array of integers arr of even length n and an integer k.

We want to divide the array into exactly n / 2 pairs such that the sum of each pair is divisible by k.

Return true If you can find a way to do that or false otherwise.

Example 1:

Input: arr = [1,2,3,4,5,10,6,7,8,9], k = 5
Output: true
Explanation: Pairs are (1,9),(2,8),(3,7),(4,6) and (5,10).

Example 2:

Input: arr = [1,2,3,4,5,6], k = 7
Output: true
Explanation: Pairs are (1,6),(2,5) and(3,4).

Example 3:

Input: arr = [1,2,3,4,5,6], k = 10
Output: false
Explanation: You can try all possible pairs to see that there is no way to divide arr into 3 pairs each with sum divisible by 10.

Constraints:

• arr.length == n
• 1 <= n <= 105
• n is even.
• -109 <= arr[i] <= 109
• 1 <= k <= 105

Solution

class Solution {
public:
  bool canArrange(vector<int>& arr, int k) {
    map<int, int> count;
    for(auto n : arr) {
      count[(n % k + k) % k] += 1;
    }
    if(count[0] % 2) return false;
    if(!(k % 2) && count[k / 2] % 2) return false;
    for(int i = 1; i < k; ++i) {
      if(count[i] != count[k - i]) return false;
    }
    return true;
  }
};