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).

 固定一个数，然后从剩余的元素中选择两个。

C++实现代码：

#include<iostream>

#include<algorithm>

#include<vector>

#include<climits>

#include<cmath>

using namespace std;



class Solution

{

public:

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

    {

        if(num.empty())

            return 0;

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

        int minSum=0;

        int close=INT_MAX;

        int sum;

        int n=num.size();

        int i;

        for(i=0; i<n-2; i++)

        {

            int left=i+1;

            int right=n-1;

            while(left<right)

            {

                if(num[i]+num[left]+num[right]<target)

                {

                    sum=num[i]+num[left]+num[right];

                    cout<<sum<<endl;

                    if(target-sum<close)

                    {

                        close=target-sum;

                        minSum=sum;

                    }

                    left++;

                }

                else if(num[i]+num[left]+num[right]>target)

                {

                    sum=num[i]+num[left]+num[right];

                    cout<<sum<<endl;

                    if(sum-target<close)

                    {

                        close=sum-target;

                        minSum=sum;

                    }

                    right--;

                }

                else if(num[i]+num[left]+num[right]==target)

                {

                    close=0;

                    //cout<<"i "<<i<<" left "<<left<<" right "<<right<<endl;

                    //cout<<num[i]<<" "<<num[left]<<" "<<num[right]<<endl;

                    minSum=num[i]+num[left]+num[right];

                    return minSum;

                }

            }

        }

        return minSum;

    }

};

int main()

{

    vector<int> vec= {1,2,3,4};

    Solution s;

    int result=s.threeSumClosest(vec,1);

    cout<<result<<endl;

}

运行结果：