[LeetCode]4Sum

class Solution {//fix i, j, then let p q walk to find out the fourSum target O(n^3)//there exists O(n^2logn) solutionpublic:vector<vector<int> > fourSum(vector<int> &num, int target) {// Start typing your C/C++ solution below// DO NOT write int main() functionif(num.size() < 4) return vector<vector<int> >();sort(num.begin(), num.end());vector<vector<int>> ans;for (int i = 0; i < num.size()-3; ++i){for (int j = i+1; j < num.size()-2; ++j){int p = j+1; int q = num.size()-1;fourSum_aux(num, target, i, j, p, q, ans);}}return ans;}void fourSum_aux( const vector<int> & num, int target, int i, int j, int p, int q,   vector<vector<int>>& ans ) {//throw std::exception("The method or operation is not implemented.");target = target-num[i]-num[j];while (p < q){int sum = num[p]+num[q];if(sum > target)q--;else if (sum < target)p++;else {vector<int> path(4);path[0] = num[i]; path[1] = num[j]; path[2] = num[p]; path[3] = num[q];p++;if ( find(ans.begin(), ans.end(), path) == ans.end() )//...ans.push_back(path);}}}};

second time

class Solution {public:    void fourSumUtil(vector<int>& num, int target, int firstIdx, int secondIdx, vector<vector<int> >& allPath)    {        target = target-num[firstIdx]-num[secondIdx];        int left = secondIdx+1;        int right = num.size()-1;        while(left < right)        {            int sum = num[left]+num[right];            if(sum < target) left++;            else if(sum > target) right--;            else            {                vector<int> path(4);                path[0] = num[firstIdx]; path[1] = num[secondIdx]; path[2] = num[left]; path[3] = num[right];                allPath.push_back(path);                left++;            }        }    }    vector<vector<int> > fourSum(vector<int> &num, int target) {        // Start typing your C/C++ solution below        // DO NOT write int main() function        sort(num.begin(), num.end());        vector<vector<int> > allPaths;        for(int i = 0; i < (int)num.size()-3; ++i)        {            for(int j = i+1; j < (int)num.size()-2; ++j)            {                fourSumUtil(num, target, i, j, allPaths);            }        }        //abort duplicates        vector<vector<int> > ans;        for(int i = 0; i < allPaths.size(); ++i)            if(find(ans.begin(), ans.end(), allPaths[i]) == ans.end()) ans.push_back(allPaths[i]);        return ans;    }};