1814. 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.

class Solution {
    int reverse(int n) {
        int res = 0;
        while(n) {
            res = res * 10 + n % 10;
            n /= 10;
        }
        return res;
    }
public:
    int countNicePairs(vector<int>& nums) {
        long long res = 0, mod = 1e9 + 7;
        string rev;
        unordered_map<int, int> m;
        for(auto& n : nums) {
            m[n - reverse(n)]++;
        }
        for(auto& [k, v] : m) {
            res = (res + (v * (v-1) / 2 % mod)) % mod;
        }
        return res;
    }
};