1814-count-nice-pairs-in-an-array.ts

/*
    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
*/

function countNicePairs(nums: number[]): number {
  nums = nums.map((num) => num - rev(num));

  let dic = new Map();
  let ans = 0;

  const mod = 10 ** 9 + 7;

  for (let i = 0; i < nums.length; i++) {
    let num = nums[i];

    if (!dic[num]) {
      dic[num] = 0;
    } else {
      ans = (ans + dic[num]) % mod;
    }

    dic[num]++;
  }

  return ans;
}

function rev(num: number): number {
  return +`${num}`.split("").reverse().join("");
}

// Result
console.log(countNicePairs([42, 11, 1, 97])); // 2