标签：style blog color io ar for art div sp
在1 - 10 中，求出 7 个数的排列组合。
出现了超时，而超时的原因是有好多重复情况，复杂度上来说，和答案的复杂度是一样的，但是答案中重复了太多了，体会下。
超时1：
class Solution { public: vector<vector<int> > combine(int n, int k) { vector<vector<int> > ans; if(n < k || n <= 0 || k <= 0) return ans; vector<int> ansPiece; set<int> checkExist; for(int i = 1; i <= n; i++) { ansPiece.clear(); ansPiece.push_back(i); checkExist.clear(); checkExist.insert(i); if(k > 1) combine(ans,ansPiece,k,n,checkExist); } return ans; } void combine(vector<vector<int> > &ans, vector<int> &ansPiece, int &k, int &n, set<int> &checkExist) { if(ansPiece.size() == k) { vector<int> temp = ansPiece; ans.push_back(temp); return; } for(int i = 1; i <=n; i++) { if(checkExist.find(i) == checkExist.end()) { ansPiece.push_back(i); checkExist.insert(i); combine(ans,ansPiece,k,n,checkExist); ansPiece.pop_back(); checkExist.erase(i); } } } };
超时2：
class Solution { public: vector<vector<int> > combine(int n, int k) { vector<vector<int> > ans; if(n < k || n <= 0 || k <= 0) return ans; vector<int> ansPiece; set<int> checkExist; //记录还没有选的元素 for(int i = 1; i <= n; i++) { checkExist.insert(i); } combine(ans,ansPiece,k,checkExist); return ans; } void combine(vector<vector<int> > &ans, vector<int> &ansPiece, int &k, set<int> &checkExist) { if(ansPiece.size() == k) { vector<int> temp = ansPiece; ans.push_back(temp); return; } set<int>::iterator itr; for(itr = checkExist.begin(); itr != checkExist.end(); itr++) { ansPiece.push_back(*itr); set<int> check2 = checkExist; check2.erase(*itr); combine(ans,ansPiece,k,check2); ansPiece.pop_back(); } } };
正确的：（控制了顺序）
class Solution { public: vector<vector<int> > combine(int n, int k) { vector<vector<int> > ans; if(n < k || n <= 0 || k <= 0) return ans; vector<int> ansPiece; int start = 1; combine(ans,ansPiece, k,n, start); return ans; } void combine(vector<vector<int> > &ans, vector<int> &ansPiece, int &k, int &n, int start) { if(ansPiece.size() == k) { vector<int> temp = ansPiece; ans.push_back(temp); return; } for(int i = start; i <= n; i++) { if(n - i + 1 < k - ansPiece.size()) return; ansPiece.push_back(i); combine(ans,ansPiece,k,n,i+1); ansPiece.pop_back(); } } };
标签：style blog color io ar for art div sp
原文地址：http://www.cnblogs.com/qingcheng/p/3959843.html
