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>& nums, int target) {       sort(nums.begin(), nums.end());       vector<vector<int>> result;       vector<int> temp;       int len = nums.size();       int i =0;       while(i <len-3){           int j = i+1;           while(j < len-2){               int k = j +1, w = len-1;               while(k <w)               {                   int sum = nums[i] + nums[j] + nums[k] + nums[w];                   if(sum==target)                   {                        temp.push_back(nums[i]);                        temp.push_back(nums[j]);                        temp.push_back(nums[k]);                        temp.push_back(nums[w]);                        result.push_back(temp);                        temp.clear();                        k++;                        w--;                         while(k < w && nums[k]==nums[k-1])                            k++;                        while(k < w && nums[w]==nums[w+1])                            w--;                   }else if(sum < target)                   {                       k++;                        while(k < w && nums[k]==nums[k-1])                            k++;                   }else                   {                       w--;                         while(k < w && nums[w]==nums[w+1])                            w--;                   }               }               j++;                while(j < len-2 && nums[j]==nums[j-1])                            j++;           }           i++;            while(i < len-3 && nums[i]==nums[i-1])                            i++;       }      return result;     }};