【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：

public class Solution {
    public int threeSumClosest(int[] num, int target) {
        if(num == null || num.length == 0)
            return 0;
        Arrays.sort(num);
        int closest = Integer.MAX_VALUE/2;
        
        for(int i = 0;i < num.length;i++){
            int start = i + 1;
            int last = num.length-1;
            while(start < last){
                int sum = num[i] + num[start] + num[last];
                if(sum == target)
                    return sum;
                else if(sum < target)
                {
                    start++;
                }
                else{
                    last--;
                }
                closest = Math.abs(closest-target) > Math.abs(sum - target)?sum:closest;
            }
        }
        
        return closest;
    }
}