dfs统计节点个数：
/** * Definition for a binary tree node. * struct TreeNode { * int val; * TreeNode *left; * TreeNode *right; * TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */ class Solution { public: int ans=0; void dfs(TreeNode* r){ if(r->left){ ans++; dfs(r->left); } if(r->right){ ans++; dfs(r->right); } } int countNodes(TreeNode* root) { if(!root) return ans; ans+=1; dfs(root); return ans; } };