3Sum Closest

Sort the array. For each index i up to n - 3, set left = i + 1 and right = n - 1. Compute the three-value sum and update best when abs(sum - target) is smaller than the current best distance. If the sum is below target, increment left to seek a larger value; if above target, decrement right; if equal, return target immediately because no closer sum exists. Sorting costs O(n log n) and the scans cost O(n squared), with O(1) extra search space. Duplicate skipping is optional for correctness because only one closest sum is returned, though it can avoid repeated work. Initialize best from an actual triplet, not an arbitrary sentinel that could overflow during subtraction. If ties require a rule, state and implement it explicitly; the common version guarantees or accepts any closest result.