3Sum Closest

https://oj.leetcode.com/problems/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).

解题思路：

解决了3Sum，这道题就简单了。首先排序，然后只需用一个变量记录目前最接近的sum即可。由于题目given只有一个最接近的解，所以也无需考虑重复解的问题了。时间复杂度同样是O(n^2)，代码如下。

public class Solution {

    public int threeSumClosest(int[] num, int target) {

        Arrays.sort(num);

        if(num.length == 0){

            return 0;

        }

        int closestSum = num[0] + num[1] + num[2];

        

        for(int i = 0; i < num.length - 2; i++){

            int start = i + 1;

            int end = num.length - 1;

            while(start < end){

                int sum = num[i] + num[start] + num[end];

                if(sum == target){

                    return target;

                }else if (sum < target){

                    start++;

                    if(Math.abs(sum - target) < Math.abs(closestSum - target)){

                        closestSum = sum;

                    }

                    

                }else if (sum > target){

                    end--;

                    if(Math.abs(sum - target) < Math.abs(closestSum - target)){

                        closestSum = sum;

                    }

                }

            }

        }

        return closestSum;

    }

}