1814. Count Nice Pairs in an Array (#162)

Author: remy727

README.md

@@ -393,6 +393,7 @@
 | 1793 | Maximum Score of a Good Subarray | [Ruby](./algorithms/ruby/1793-maximum-score-of-a-good-subarray.rb) | Hard |
 | 1799 | Maximize Score After N Operations | [Ruby](./algorithms/ruby/1799-maximize-score-after-n-operations.rb) | Hard |
 | 1802 | Maximum Value at a Given Index in a Bounded Array | [Ruby](./algorithms/ruby/1802-maximum-value-at-a-given-index-in-a-bounded-array.rb) | Medium |
+| 1814 | Count Nice Pairs in an Array | [Ruby](./algorithms/ruby/1814-count-nice-pairs-in-an-array.rb) | Medium |
 | 1822 | Sign of the Product of an Array | [Ruby](./algorithms/ruby/1822-sign-of-the-product-of-an-array.rb) | Easy |
 | 1838 | Frequency of the Most Frequent Element | [Ruby](./algorithms/ruby/1838-frequency-of-the-most-frequent-element.rb) | Medium |
 | 1845 | Seat Reservation Manager | [Ruby](./algorithms/ruby/1845-seat-reservation-manager.rb) | Medium |

algorithms/ruby/1814-count-nice-pairs-in-an-array.rb

@@ -0,0 +1,52 @@
+# frozen_string_literal: true
+
+# 1814. Count Nice Pairs in an Array
+# Medium
+# https://leetcode.com/problems/count-nice-pairs-in-an-array
+
+=begin
+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
+=end
+
+# @param {Integer[]} nums
+# @return {Integer}
+def count_nice_pairs(nums)
+  y = Hash.new { |h, x| h[x] = x - x.to_s.reverse.to_i }
+  res = 0
+  c = Hash.new(0)
+  for x in nums
+    res += c[y[x]]
+    c[y[x]] += 1
+  end
+  res % 1000000007
+end
+
+# **************** #
+#       TEST       #
+# **************** #
+
+require "test/unit"
+class Test_count_nice_pairs < Test::Unit::TestCase
+  def test_
+    assert_equal 2, count_nice_pairs([42, 11, 1, 97])
+    assert_equal 4, count_nice_pairs([13, 10, 35, 24, 76])
+  end
+end