Leetcode#18 4Sum

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Given an array S of n integers, are there elements abc, 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)

Difficulty: Medium

思路不是很难,可是Debug真是一件无比痛!苦!的事,足足花了我两个小时,可能还是不够熟练吧。

vector<vector<int> > fourSum(vector<int>& nums, int target) {       vector<vector<int> > ans;       int len = nums.size();       if(len<4)        return ans;        sort(nums.begin(),nums.end());       for(int p1 = 0; p1<len-3;p1++)      {        for(int p4 = p1+1; p4<len-2; p4++)       {           int p2 = p4+1, p3 = len-1;           if(nums[p4]==nums[p4-1]&&p4-1>p1)            continue;        if(nums[p1]==nums[p1-1]&&p1>0)            continue;           while(p3>p2)           {               int target2 = target - nums[p1] - nums[p4];               if(nums[p2]==nums[p2-1]&&p2-1>p4)               {                   p2++;                   continue;               }               if(nums[p3]==nums[p3+1]&&p3+1<len-1)               {                   p3--;                   continue;               }               if(nums[p3]+nums[p2]>target2)               {                   p3--;                   continue;               }               else if(nums[p3]+nums[p2]<target2)               {                   p2++;                   continue;               }               else               {                   vector<int> tmp;                   int test;                    tmp.push_back(nums[p1]);                    tmp.push_back(nums[p4]);                    tmp.push_back(nums[p2]);                    tmp.push_back(nums[p3]);                    ans.push_back(tmp);                    p2++;                    continue;               }           }       }}        return ans;    }



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