4Sum

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)

思路同3Sum

vector<vector<int> > fourSum(vector<int> &num, int target) {        vector<vector<int> > vecall;        vector<int> vec;        sort(num.begin(), num.end());        for(int i = 0; i < (int)num.size() - 3; i++){            if(i > 0 && num[i] == num[i-1])                continue;            for(int j = i + 1; j < (int)num.size() - 2; j++){                if(j > i +1 && num[j] == num[j-1])                    continue;                int low = j + 1, high = num.size() - 1;                while(low < high){                    if(low > j+1 && num[low] == num[low-1]){                        low++;                        continue;                    }                    if(high < num.size() - 1 && num[high] == num[high+1]){                        high--;                        continue;                    }                    int check = num[i] + num[j] + num[low] + num[high];                    if(check == target){                        vec.push_back(num[i]);                        vec.push_back(num[j]);                        vec.push_back(num[low]);                        vec.push_back(num[high]);                        vecall.push_back(vec);                        vec.clear();                        low++, high--;                    }else if(check > target)                        high--;                    else low++;                }            }        }        return vecall;    }