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: Similar to 3Sum two pointers solution.

1. Sort the array
2. Initialize three indexes i, j, k, where i traverse from 0 to the end, j starts from i + 1 position to k k starts from the end of the array to j
3. Calculate the sum, and difference if get the smallest difference, update the so far closest value and difference value. if sum > target : k--

     < target : j++
     = target : return target
   

4. $$O(n^2)$$

public class Solution {
    public int threeSumClosest(int[] nums, int target) {
        Arrays.sort(nums);
        int diff = Integer.MAX_VALUE;
        int res = 0;

        for(int i = 0; i < nums.length; i++){
            int j = i + 1;
            int k = nums.length - 1;

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

                if(sum == target){
                    return target;
                }else if (sum < target){
                    j++;
                }else{
                    k--;
                }

            }
        }
            return res;

    }
}