Count Pairs With Target Sum

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Given an integer array nums and an integer target, return how many index pairs (i, j) with i < j satisfy nums[i] + nums[j] == target. Pairs with equal values at different positions count separately.

Example

nums = [3,3,3], target = 6  ->  3

(0,1), (0,2) and (1,2) each form a pair.

Constraints

• 0 <= nums.length <= 10^4
• -10^4 <= nums[i], target <= 10^4

Similar to LC 1 (Two Sum), which returns one pair instead of counting all of them.