代码随想录算法训练营第二十一天| 77. 组合

回溯——组合#

思路：#

套了K次的循环，如何对K次循环呢，就需要一个startIndex，用它来控制从哪里开始

代码：#

 1 void combine_backTrack(int n, int k, int startIndex, vector<int>& path, vector<vector<int>>& result)
 2 {
 3     //中止条件
 4     if (path.size() == k)
 5     {
 6         result.push_back(path);
 7         return;
 8     }
 9 
10     for (int i = startIndex; i <= n; i++)
11     {
12         path.push_back(i);
13         combine_backTrack(n, k, i + 1, path, result);
14         path.pop_back();
15     }
16 }
17 
18 vector<vector<int>> combine(int n, int k) {
19     vector<vector<int>> result;
20     if (n == 0) return result;
21     vector<int> path;    
22     combine_backTrack(n, k, 1, path, result);
23     return result;
24 }

剪枝操作：#

需要n-(k-p.size())+1

代码：#

 1 void combine_backTrack(int n, int k, int startIndex, vector<int>& path, vector<vector<int>>& result)
 2 {
 3     //中止条件
 4     if (path.size() == k)
 5     {
 6         result.push_back(path);
 7         return;
 8     }
 9 
10     for (int i = startIndex; i <= (n-(k-path.size())+1); i++)
11     {
12         path.push_back(i);
13         combine_backTrack(n, k, i + 1, path, result);
14         path.pop_back();
15     }
16 }
17 
18 vector<vector<int>> combine(int n, int k) {
19     vector<vector<int>> result;
20     if (n == 0) return result;
21     vector<int> path;    
22     combine_backTrack(n, k, 1, path, result);
23     return result;
24 }