77. 组合
今天就做一题，回溯剪枝。我的理解就是把多层for循环放到递归里实现，通过剪枝来减少递归次数。遍历顺序类似N叉树的遍历。
class Solution {
public://定义全局变量更方便vector<int> path;vector<vector<int>> res;void backtracking(int n, int k, int startIndex){if (path.size() == k){res.push_back(path);return;}// 未剪枝// for (int i = startIndex;i <= n; i++){// path.push_back(i);// backtracking(n, k, i + 1);// path.pop_back();// }// 剪枝for (int i = startIndex; i <= n - (k - path.size()) + 1; i++){path.push_back(i);backtracking(n, k, i + 1);path.pop_back();}}vector<vector<int>> combine(int n, int k) {backtracking(n, k, 1);return res;}
};
)
