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

November LeetCoding Challenge 21

Description

Count Nice Pairs in an Array

You are given an array nums that consists of non-negative integers. Let us define rev(x) as the reverse of the non-negative integer x. For example, rev(123) = 321, and rev(120) = 21. A pair of indices (i, j) is nice if it satisfies all of the following conditions:

• 0 <= i < j < nums.length
• nums[i] + rev(nums[j]) == nums[j] + rev(nums[i])

Return the number of nice pairs of indices. Since that number can be too large, return it modulo 109 + 7.

Example 1:

Input: nums = [42,11,1,97]
Output: 2
Explanation: The two pairs are:
 - (0,3) : 42 + rev(97) = 42 + 79 = 121, 97 + rev(42) = 97 + 24 = 121.
 - (1,2) : 11 + rev(1) = 11 + 1 = 12, 1 + rev(11) = 1 + 11 = 12.

Example 2:

Input: nums = [13,10,35,24,76]
Output: 4

Constraints:

• 1 <= nums.length <= 105
• 0 <= nums[i] <= 109

Solution

auto speedup = [](){
  cin.tie(nullptr);
  cout.tie(nullptr);
  ios::sync_with_stdio(false);
  return 0;
}();
class Solution {
  const int MOD = 1e9 + 7;
  int rev(int num) {
    int result = 0;
    while(num) {
      result *= 10;
      result += num % 10;
      num /= 10;
    }
    return result;
  }
public:
  int countNicePairs(vector<int>& nums) {
    map<int, int> revDiff;
    for(auto n : nums) {
      revDiff[n - rev(n)] += 1;
    }
    int answer = 0;
    for(const auto &[_diff, count] : revDiff) {
      answer += 1LL * count * (count - 1) / 2 % MOD;
      answer %= MOD;
    }
    return answer;
  }
};

// Accepted
// 84/84 cases passed (71 ms)
// Your runtime beats 99.16 % of cpp submissions
// Your memory usage beats 98.74 % of cpp submissions (56.2 MB)