Given a list nums, count the index pairs (i, j) with i < j and nums[i] == nums[j].
Examples
Input: nums = [2, 2, 2, 5, 5]
Output: 4
Explanation: three pairs of 2s (0,1), (0,2), (1,2) and one pair of 5s (3,4).
Input: nums = [1, 2, 3]
Output: 0
Constraints
0 <= len(nums) <= 10**5- The answer can exceed 2**31; Python ints are fine.
- Target complexity: O(n) time. A double loop is too slow.
Goals
- Count pairs without a nested loop
- Use the count-so-far trick while scanning