1814. Count Nice Pairs in an Array

Problem Link

Description

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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

Solution

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Python3

class Solution:
    def countNicePairs(self, nums: List[int]) -> int:
        N = len(nums)
        mp = defaultdict(int)
        MOD = 10 ** 9 + 7
        res = 0
 
        def rev(x):
            return int(str(x)[::-1])
 
 
        # nums[i] + rev(nums[j]) == nums[j] + rev(nums[i])
        # nums[i] - rev(nums[i]) == nums[j] - rev(nums[j])
        for x in nums:
            res += mp[x - rev(x)]
            mp[x - rev(x)] += 1
        
        return res % MOD