4Sum 4个数字的和为目标数

4Sum

Given an array S of n integers, are there elements a, b, c, and d in S such
that a + b + c + d = target? Find all unique quadruplets in the array which gives the sum of target.
Note:

Elements in a quadruplet (a,b,c,d) must be in non-descending order. (ie, a ≤ b ≤ c ≤ d)
The solution set must not contain duplicate quadruplets.

For example, given array S = {1 0 -1 0 -2 2}, and target = 0.

    A solution set is:
    (-1,  0, 0, 1)
    (-2, -1, 1, 2)
    (-2,  0, 0, 2)

class Solution {
public:
    vector<vector<int> > fourSum(vector<int> &num, int target) {
        
        vector<vector<int>> res;
        int i,j,left,right,len,sum;
        len=num.size();
        sort(num.begin(),num.end());
        for(i=0;i<len;i++)
        {
            if(i>0&&num[i]==num[i-1])
            {
                continue;
            }
            for(j=i+1;j<len;j++)
            {
                if(j>i+1&&num[j]==num[j-1])
                {
                    continue;
                }
                left=j+1;
                right=len-1;
                while(left<right)
                {
                    if(left>j+1&&num[left]==num[left-1])
                    {
                        left++;
                        continue;
                    }
                    if(right<len-1&&num[right]==num[right+1])
                    {
                        right--;
                        continue;
                    }
                    sum=num[i]+num[j]+num[left]+num[right];
                    if(sum<target)
                    {
                        left++;
                    }
                    else if(sum>target)
                    {
                        right--;
                    }
                    else{
                        vector<int> tmp;
                        tmp.push_back(num[i]);
                        tmp.push_back(num[j]);
                        tmp.push_back(num[left]);
                        tmp.push_back(num[right]);
                        res.push_back(tmp);
                        left++;
                        right--;
                    }
                }
            }
        }
        return res;
    }
};