Question:

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).

Thinking:

In order to improve the performance of algorithm, we should sort the array. Because of that, we can determine where to go using current result.

Solution:

public int threeSumClosest(int[] nums, int target) {
    int l = nums.length;
    if (l < 3)
        return 0;
    int min = Integer.MAX_VALUE;
    int res = 0;
    Arrays.sort(nums);

    for (int i = 0; i < l; i++){
        int j = i + 1;
        int k = l - 1;

        while (j < k){
            int sum = nums[i] + nums[j] + nums[k];
            if (sum == target)
                return sum;
            int diff = Math.abs(sum - target);
            if (diff < min){
                min = diff;
                res = sum;
            }
            if (sum > target)
                k--;
            else
                j++;
        }

    }
    return res;
}