1512 Number of Good Pairs - Easy

Problem:

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

Problem Analysis:

• use hashmap
• O(n)

Solutions:

class Solution:
    def numIdenticalPairs(self, nums: List[int]) -> int:
        res = 0
        count = {}

        for n in nums:
            if n in count:
                res+= count[n]
                count[n] +=1
            else:
                count[n] =1

        return res        

class Solution:
    def numIdenticalPairs(self, nums: List[int]) -> int:
        count = {}
        res = 0

        for n in nums:
            count[n] = count.get(n, 0) + 1
            res += count[n] - 1

        return res

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