334. Number of Good Pairs

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:
Input: nums = [1,2,3,1,1,3]
Output: 4

Explanation: There are 4 good pairs (0,3), (0,4), (3,4), (2,5).

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