How should I practice Count Index Pairs with a Target Sum?

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Count Index Pairs with a Target Sum

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Problem statement

Return the number of index pairs (i,j) with i < j and nums[i] + nums[j] == target. Equal values at different indices form distinct pairs.

Function

countTargetPairs(nums: int[], target: int) → long

Examples

Example 1

nums = [1,2,1,1,2]target = 3return = 6

Each of three 1s pairs with each of two 2s.

Constraints

0 <= nums.length <= 2 * 10^5.
The answer fits in signed 64-bit range.