16. 3Sum Closest

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).

Solution:

Sort the array and use two pointers to find the sum of three numbers.

We maintain a closest number to target. Each time if we find the sum is closer, we update the closest number.

Time complexity is O(n²).



Code:

public class Solution {
    public int threeSumClosest(int[] nums, int target) {
        if (nums == null || nums.length < 3) {
            return -1;
        }
        Arrays.sort(nums);
        int closest = nums[0] + nums[1] + nums[2];
        for (int i = 0; i < nums.length - 2; i++) {
            int left = i + 1;
            int right = nums.length - 1;
            while (left < right) {
                int sum = nums[i] + nums[left] + nums[right];
                if (Math.abs(sum - target) < Math.abs(closest - target)) {
                    closest = sum;
                }
                if (sum < target) {
                    left++;
                }
                else {
                    right--;
                }
            }
        }
        return closest;
    }
}