How hard is LeetCode 1814. Count Nice Pairs in an Array?

LeetCode 1814. Count Nice Pairs in an Array is rated Medium on LeetCode.

What is the time complexity of LeetCode 1814. Count Nice Pairs in an Array?

The Python solution on this page runs in O(n \times \log M), where n and M are the length of the nums array and the maximum value in the nums array, respectively.

What is the space complexity of LeetCode 1814. Count Nice Pairs in an Array?

The Python solution on this page uses O(n) auxiliary space.

What topics does LeetCode 1814. Count Nice Pairs in an Array cover?

LeetCode 1814. Count Nice Pairs in an Array is tagged Array, Hash Table, Math and Counting on LeetCode.

Count Nice Pairs in an Array — LeetCode 1814 Python Solution

MediumArrayHash TableMathCounting

Problem: #1814
Pattern: Math and Number Theory
Reading time: 2 min
Source: leetcode.com

The problem

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.

Example

Input: nums = [42,11,1,97]
Output: 2
Explanation: The two pairs are:

Python solution

Python

class Solution:
    def countNicePairs(self, nums: List[int]) -> int:
        def rev(x):
            y = 0
            while x:
                y = y * 10 + x % 10
                x //= 10
            return y

        cnt = Counter(x - rev(x) for x in nums)
        mod = 10**9 + 7
        return sum(v * (v - 1) // 2 for v in cnt.values()) % mod

Complexity

Measure	Complexity
Time	O(n \times \log M), where n and M are the length of the nums array and the maximum value in the nums array, respectively
Space	O(n) auxiliary

Pattern: Math and Number Theory

Find the closed form, the invariant, or the modular identity — and skip the loop entirely. LeetCode 1814. Count Nice Pairs in an Array is filed here because LeetCode tags it Math, which is the vocabulary this hub collects.

The math and number theory guide has the Python template for the pattern and the 485 LeetCode problems that use it.

Frequently asked questions

How hard is LeetCode 1814. Count Nice Pairs in an Array?: LeetCode 1814. Count Nice Pairs in an Array is rated Medium on LeetCode.
What is the time complexity of LeetCode 1814. Count Nice Pairs in an Array?: The Python solution on this page runs in O(n \times \log M), where n and M are the length of the nums array and the maximum value in the nums array, respectively.
What is the space complexity of LeetCode 1814. Count Nice Pairs in an Array?: The Python solution on this page uses O(n) auxiliary space.
What topics does LeetCode 1814. Count Nice Pairs in an Array cover?: LeetCode 1814. Count Nice Pairs in an Array is tagged Array, Hash Table, Math and Counting on LeetCode.