【leetcode】3SumClosest

题目：

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：和3SUM差不多，先用排序可以降低时间复杂度，排序后用O(n^2)就可以找出，即先假定一个元素在里面，找另外两个使得差最小。

代码如下：

import java.util.Arrays;

/**
* Three Sum Closest
* @author JeremyCai
*
*/
public class ThreeSumClosest {

/**
* 先用排序，后用两重循环，第一重是假定一个元素在里面，里面一重循环是找到另外两个数使得和最接近
* @param num
* @param target
* @return
*/
public int threeSumClosest(int[] num, int target) {
Arrays.sort(num);
int minMargin = num[0] + num[1] + num[2] - target;
for(int i = 0; i < num.length - 2; i++){
for(int j = i + 1, k = num.length - 1;j < k;){
int temp = num[j] + num[k] + num[i];
if(Math.abs(minMargin) > Math.abs(temp - target))
minMargin = temp - target;
if(temp == target)
return target;
else if(temp < target)
j++;
else
k--;
}
}
return minMargin + target;
}

public static void main(String[] args) {
int num[] = {-1,2,1,-4};
System.out.println(new ThreeSumClosest().threeSumClosest(num, 1));
}
}