3Sum Closest

【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).

【分析】

1.思路与3 Sum类似，采用两层循环的方式。

2.需要记录当前3个值的和与target的差的最小值

【算法实现】

public class Solution {
    public int threeSumClosest(int[] num, int target) {
        Arrays.sort(num);
        if(num.length<3)
            return 0;
        int res=num[0]+num[1]+num[2];
        for(int i=0;i<num.length;i++) {
            int start=i+1;
            int end=num.length-1;
            while(start<end) {
                int sum=num[i]+num[start]+num[end];
                if(Math.abs(sum-target)<Math.abs(res-target))
                    res=sum;
                else if(sum==target)
                    return sum;
                else if(sum>target)
                    end--;
                else 
                    start++;
            }
        }
        return res;
    }
}