/** * Definition for binary tree * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    vector
     
       preorderTraversal(TreeNode *root) {        vector
      
        res;        if (root == NULL) return res;         vector
       
         stack;        stack.push_back(root);        while(!stack.empty()){             TreeNode* n = stack.back();            stack.pop_back();            res.push_back(n->val);            if (n->right != NULL) stack.push_back(n->right);            if (n->left != NULL) stack.push_back(n->left);        }    }};
       
      
     

水一发，非递归通用实现见