Number of Good Pairs

LeetCode 1635 | Difficulty: Easy

Easy

Problem Description

Given an array of integers nums, return *the number of good pairs*.

A pair (i, j) is called good if nums[i] == nums[j] and i < j.

Example 1:

Input: nums = [1,2,3,1,1,3]
Output: 4
Explanation: There are 4 good pairs (0,3), (0,4), (3,4), (2,5) 0-indexed.

Example 2:

Input: nums = [1,1,1,1]
Output: 6
Explanation: Each pair in the array are good.

Example 3:

Input: nums = [1,2,3]
Output: 0

Constraints:

- `1 <= nums.length <= 100`

- `1 <= nums[i] <= 100`

Topics: Array, Hash Table, Math, Counting

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Approach

Hash Map

Use a hash map for O(1) average lookups. Store seen values, frequencies, or indices. The key question: what should I store as key, and what as value?

When to use

Need fast lookups, counting frequencies, finding complements/pairs.

Mathematical

Look for mathematical patterns or formulas. Consider: modular arithmetic, GCD/LCM, prime factorization, combinatorics, or geometric properties.

When to use

Problems with clear mathematical structure, counting, number properties.

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Solutions

Solution 1: C# (Best: 76 ms)

Metric	Value
Runtime	76 ms
Memory	37.2 MB
Date	2021-12-29

Solution

public class Solution {
    public int NumIdenticalPairs(int[] nums) {
        int[] times = new int[101];
    for (int i = 0; i < nums.Length; i++)
    {
        times[nums[i]]++;
    }
    int count = 0;
    for (int i = 0; i < 101; i++)
    {
        if(times[i] <= 1) continue;
        int pairs = (times[i]-1) * (times[i])/2;
        count += pairs;
    }
    return count;
    }
}

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Complexity Analysis

Approach	Time	Space
Hash Map	$O(n)$	$O(n)$

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Interview Tips

Key Points

• Start by clarifying edge cases: empty input, single element, all duplicates.
• Hash map gives O(1) lookup — think about what to use as key vs value.
• LeetCode provides 1 hint(s) for this problem — try solving without them first.

💡 Hints

Hint 1: Count how many times each number appears. If a number appears n times, then n * (n – 1) // 2 good pairs can be made with this number.