Count Unique Pairs with Sum Equal to Target

amazon Given an integer array array and an integer target, calculate the number of unique pairs that sum to target. Note that a pair is considered as a set, so the order does not matter. For example, if array is [1,2,1] and target is 3, the valid pair is (1,2), so the result is 1. Input array: An integer array. target: An integer. Output Return the number of unique pairs that sum to target. Example array = [1, 2, 1] target = 3 Output: 1 Constraints Array length does not exceed 10^4. Absolute values of array elements do not exceed 10^4. Absolute values of target do not exceed 10^5. Example Input 1 3 0