preorder 遍历二叉树
/**
* 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:
vector<int> preorderTraversal(TreeNode* root) {
vector<int> res;
stack<TreeNode *> s;
if (root == NULL) return res;
s.push(root);
while (!s.empty()) {
TreeNode *node = s.top();
s.pop();
res.push_back(node->val);
if (node->right != NULL) s.push(node->right);
if (node->left != NULL) s.push(node->left);
}
return res;
}
};
