1814. Count Nice Pairs in an Array

Author: cheehwatang

solutions/1814. Count Nice Pairs in an Array/CountNicePairsInAnArray.java

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+package com.cheehwatang.leetcode;
+
+import java.util.HashMap;
+import java.util.Map;
+
+// Time Complexity  : O(n),
+// where 'n' is the length of 'nums'.
+// We traverse 'nums' to determine and count the relative values.
+//
+// Space Complexity : O(n),
+// where 'n' is the length of 'nums'.
+// We use a Hash Table to count the relative values, with a maximum size of 'n'.
+
+public class CountNicePairsInAnArray {
+
+    // Approach:
+    // First we need to understand the bad pair, i < j and nums[i] + rev(nums[j]) == nums[j] + rev(nums[i]).
+    // With the brute force, we can check each and every pair if it is a nice pair,
+    // but the algorithm will exceed the time limit.
+    // Thus, let's change the equation for the bad pair as follows:
+    // nums[i] - rev(nums[i]) == nums[j] - rev(nums[j]).
+    // Use a HashMap to record the nums[i] - rev(nums[i]) for all the numbers, and the frequency.
+
+    public int countNicePairs(int[] nums) {
+
+        // Use long type, so as not the keep doing the modulo function, only need to do once at the end.
+        long nicePairs = 0;
+        Map<Integer, Integer> map = new HashMap<>();
+
+        for (int number : nums) {
+            // Calculate the difference (relative value) between the nums[i] and rev(nums[i]).
+            int relative = number - reverse(number);
+            // If the map already had numbers with the same relative value, then add the frequency as the nice pairs.
+            nicePairs += map.getOrDefault(relative, 0);
+            // Increase the count in the map.
+            map.put(relative, map.getOrDefault(relative, 0) + 1);
+        }
+        return (int) (nicePairs % (1e9 + 7));
+    }
+
+    // Method to reverse the number.
+    private int reverse(int number) {
+        int reverse = 0;
+        while (number > 0) {
+            reverse = reverse * 10 + number % 10;
+            number /= 10;
+        }
+        return reverse;
+    }
+}