LeetCode OJ--4Sum *

https://oj.leetcode.com/problems/4sum/

在一个数列中，找出所有的4个数，它们的和是target.

class Solution {
public:
    vector<vector<int> > fourSum(vector<int> &num, int target)  {
        vector<vector<int> > ans;
        if(num.size()<4)
            return ans;

        sort(num.begin(),num.end());
        for(int i = 0;i<num.size()-3;i++)
        {
            if(i>0 && num[i]==num[i-1])
                continue;
             
            calcThree(num,target - num[i],i,ans);
        }
        return ans;
    }
    void calcThree(vector<int> &num,int threeSum,int firstPosition,vector<vector<int> > &ans)
    {
        int k = 0;

        for(int i = firstPosition+1;i<num.size()-2;i++)
        {
            if(i>firstPosition+1 && num[i] == num[i-1])
                continue;
            int j = i+1;
 
            k = num.size()-1;
            
            while(j<k)
            {
                if(num[i]+num[j]+num[k] == threeSum)
                {
                    if(ans.size()==0 || ans.size()>0 && !(num[firstPosition] == ans[ans.size()-1][0] && num[i]==ans[ans.size()-1][1]&& num[j] ==ans[ans.size()-1][2] ))
                    {
                        vector<int> ansPiece;
                        ansPiece.push_back(num[firstPosition]);
                        ansPiece.push_back(num[i]);
                        ansPiece.push_back(num[j]);
                        ansPiece.push_back(num[k]);                    
                        ans.push_back(ansPiece);
                    }
                    j++;
                }
                else if(num[i]+num[j]+num[k] < threeSum)
                    j++;
                else
                    k--;
            }
        }
    }
};