3Sum - Reduce to Two Sum

Sort the array first. Then fix one element and use two pointers to find the other two.

$1.$ Sort the array.

$2.$ For each index $i$, find pairs in nums[i+1:] that sum to -nums[i].

$3.$ Use two pointers (like Two Sum II) for the pair search.

$4.$ Skip duplicates: if nums[i] == nums[i-1], skip. Similarly skip duplicate left/right values.