【leetcode刷题笔记】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).

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题解：3Sum类似，只要维护一个变量closest作为当前和target最近的和。如果之间能够凑出来target，返回target就可以了。

有一个坑就是closest初始化的时候不能初始化为Integer.MAXVALUE，否则closest-target当target<0的时候会溢出。

比如输入 [-3,-2,-5,3,-4], -1 Integer.MAXVALUE-(-1)瞬间就溢出了。

代码如下：

 1 public class Solution {

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

 3         if(num == null || num.length == 0)

 4             return 0;

 5         Arrays.sort(num);

 6         int closest = Integer.MAX_VALUE/2;

 7         

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

 9             int start = i + 1;

10             int last = num.length-1;

11             while(start < last){

12                 int sum = num[i] + num[start] + num[last];

13                 if(sum == target)

14                     return sum;

15                 else if(sum < target)

16                 {

17                     start++;

18                 }

19                 else{

20                     last--;

21                 }

22                 closest = Math.abs(closest-target) > Math.abs(sum - target)?sum:closest;

23             }

24         }

25         

26         return closest;

27     }

28 }