[Leetcode] 4 Sum [Unsolved]

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 â‰¤ b â‰¤ c â‰¤ 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)

DFS！用一个 Used Vector 避免重复。

大集合没过，目测应该用 DFS + Two Sum 的方法。

class Solution {public:    vector<vector<int> > fourSum(vector<int> &num, int target) {        // Start typing your C/C++ solution below        // DO NOT write int main() function        vector<vector<int> > solution;        vector<int> answer;        vector<bool> used (num.size(),false);        sort(num.begin(),num.end());        dfs(used,solution,answer,num,0,0,target);        return solution;            }        void dfs (vector<bool> &used, vector<vector<int> > &solution, vector<int> &answer, vector<int> &num, int depth, int index, int target)    {        if(depth==4)        {            if(target==0)            {                solution.push_back(answer);            }            return;        }                for(int i= index; i<num.size(); i++)        {                        if(i!=0 && num[i]==num[i-1] && used[i-1]==false)            {                            }            else            {                answer.push_back(num[i]);                used[i] = true;                dfs(used,solution,answer,num,depth+1,i+1,target-num[i]);                answer.pop_back();                used[i] = false;                            }                    }     }};