leetcode[18]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;

        if(num.size()<4)return res;

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

        int c,d;

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

        {

            if(a>0&&num[a]==num[a-1])

            continue;

            for(int b=a+1;b<num.size();b++)

            {

                if(b>a+1&&num[b]==num[b-1])

                continue;

                c=b+1;

                d=num.size()-1;

                while(c<d)

                {

                    if(c>b+1&&num[c]==num[c-1])

                    {

                        c++;

                        continue;

                    }

                    if(d<num.size()-1&&num[d]==num[d+1])

                    {

                        d--;

                        continue;

                    }

                    int sum=num[a]+num[b]+num[c]+num[d];

                    if(sum<target)c++;

                    else if(sum>target)d--;

                    else

                    {

                        vector<int> temp;

                        temp.push_back(num[a]);

                        temp.push_back(num[b]);

                        temp.push_back(num[c]);

                        temp.push_back(num[d]);

                        res.push_back(temp);

                        c++;

                        d--;

                    }

                }

            }

        }

        return res;

    }

};