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:    static bool compare_func(const int d1, const int d2) {          return d1 < d2;      }    vector<vector<int> > fourSum(vector<int> &num, int target) {         int i, j, k, l;           vector< vector<int> > data;           vector<int> element(4);           sort(num.begin(), num.end(), compare_func);           if(num.size() < 4) return data;         for(i = 0; i < num.size() - 1; i++) {              if(i != 0 && num[i] == num[i - 1]) continue;               for(l = i + 1; l < num.size(); ) {                 for(j = l + 1, k = num.size() - 1; j < k ; ) {                       if(num[j] + num[k] + num[i] + num[l] < target) {                           j++;                       }                       else if(num[j] + num[k] + num[i] + num[l] > target) {                           k--;                       }                       else {                           element[0] = num[i];                         element[1] = num[l];                         element[2] = num[j];                           element[3] = num[k];                           data.push_back(element);                           while(num[++j] == num[j - 1] && j < k);                                while(num[--k] == num[k + 1] && j < k);                         }                   }                 while(num[++l] == num[l - 1] && l < num.size());                 }         }           return data;      }};