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;
    }
}