3Sum Closest

Problem (restated)

Given an integer array nums and integer target, find three integers whose sum is closest to target. Return that sum.

Intuition

Sort, fix one index, two-pointer the rest while tracking the closest sum. same spine as 3Sum.

Approaches

export function threeSumClosest(nums: number[], target: number): number {
  nums = [...nums].sort((a, b) => a - b);
  let best = nums[0]! + nums[1]! + nums[2]!;
  for (let i = 0; i < nums.length - 2; i++) {
    let lo = i + 1, hi = nums.length - 1;
    while (lo < hi) {
      const s = nums[i]! + nums[lo]! + nums[hi]!;
      if (Math.abs(s - target) < Math.abs(best - target)) best = s;
      if (s === target) return s;
      if (s < target) lo++; else hi--;
    }
  }
  return best;
}

export function threeSumClosest(nums: number[], target: number): number {
  nums = [...nums].sort((a, b) => a - b);
  let best = nums[0]! + nums[1]! + nums[2]!;
  for (let i = 0; i < nums.length - 2; i++) {
    let lo = i + 1, hi = nums.length - 1;
    while (lo < hi) {
      const s = nums[i]! + nums[lo]! + nums[hi]!;
      if (Math.abs(s - target) < Math.abs(best - target)) best = s;
      if (s === target) return s;
      if (s < target) lo++; else hi--;
    }
  }
  return best;
}

Template connection

Sort + two pointers is the 3Sum spine; track the closest sum instead of collecting exact zeros.

Reflection

• Same skeleton as 3Sum. The target is not 0, and you record the sum, not the triple.
• Do you update abs(sum - target) on every L, R pair, or only on an exact hit?
• Is an early return legal when sum == target?