LeetCode 016 3Sum Closest

题目描述：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).

 

分析：

和3Sum很像，设一个gap，每次都比较更新gap！

① gap == 0，则返回，因为题目说明只有一个解；

② gap > 0，q--；

③ gap < 0，p++；

④ 直到p == q。

 

代码如下：

class Solution {

public:

    int threeSumClosest(vector<int> &num, int target) {

        

        int result = 0;

        int min_gap = INT_MAX;

        

        sort(num.begin(), num.end());

        

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

            

            int p = i + 1, q = num.size() - 1;

            

            while(p < q){

                int sum = num[i] + num[p] + num[q];

                int gap = abs(sum - target);

                

                if(gap < min_gap){

                    result = sum;

                    min_gap = gap;

                }

                

                if(sum < target) 

                    p++;

                else 

                    q--;

            }

        }

        

        return result;

        

    }

};