Create 1814. Count Nice Pairs in an Array (#349)

1814. Count Nice Pairs in an Array

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+class Solution {
+public:
+    // time/space: O(nlogk)/O(n)
+    int countNicePairs(vector<int>& nums) {
+        // count the difference for each number with its reverse
+        unordered_map<int, int> hash;
+        for (auto& num : nums) hash[num - rev(num)]++;
+
+        // count the nice pairs
+        int count = 0;
+        for (auto& [key, value] : hash) {
+            long long pairs = ((1LL * value * (value - 1)) / 2LL) % MOD;
+            count = (count + pairs) % MOD;
+        }
+        return count;
+    }
+private:
+    const int MOD = 1e9 + 7;
+    int rev(int x) {
+        int result = 0;
+        while (x > 0) {
+            result = 10 * result + (x % 10);
+            x /= 10;
+        }
+        return result;
+    }
+};