Problem

Given an array S of n integers, find three integers in S such that the sum is closest to a given number, target. Return the sum of the three integers. You may assume that each input would have exactly one solution.

For example, given array S = {-1 2 1 -4}, and target = 1.

The sum that is closest to the target is 2. (-1 + 2 + 1 = 2).

Difficulty:Medium

Category:Array

Solution

class Solution {
 public:
  int threeSumClosest(vector<int>& nums, int target) {
    if (nums.size() < 3) return 0;
    int closest = nums[0] + nums[1] + nums[2];
    int diff = abs(closest - target);
    sort(nums.begin(), nums.end());
    for (int i = 0; i < nums.size() - 2; ++i) {
      int left = i + 1, right = nums.size() - 1;
      while (left < right) {
        int sum = nums[i] + nums[left] + nums[right];
        int newDiff = abs(sum - target);
        if (diff > newDiff) {
          diff = newDiff;
          closest = sum;
        }
        if (sum < target)
          left++;
        else
          right--;
      }
    }
    return closest;
  }
};