1814. Count Nice Pairs in an Array

https://leetcode.com/problems/count-nice-pairs-in-an-array

Description

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

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