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> >result;        vector<int>tmp;        int sum = num.size();        if(sum < 4){            return result;        }        sort(num.begin(),num.end());        for(int i = 0; i < num.size() - 3; i++){            if(i > 0 && num[i] == num[i - 1]){                continue;            }            for(int j = i + 1; j < num.size() - 2; j++){                if(j > i + 1 && num[j] == num[j - 1]){                    continue;                }                int m = j + 1;                int n = num.size() - 1;                while(m < n){                    if(m > j + 1 && num[m] == num[m - 1]){                        m++;                        continue;                    }                    if(n < num.size() - 1 && num[n] == num[n + 1]){                        n--;                        continue;                    }                    int temp = num[i] + num[j] + num[m] + num[n];                    if(temp > target){                        n--;                    }                    else if(temp < target){                        m++;                    }                    else{                        tmp.clear();                        tmp.push_back(num[i]);                        tmp.push_back(num[j]);                        tmp.push_back(num[m]);                        tmp.push_back(num[n]);                        result.push_back(tmp);                        m++;                        n--;                    }                }            }        }        return result;    }};