leetcode 日经贴，Cpp code -Binary Tree Preorder Traversal

/**
* 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> preorder;
stack<TreeNode*> st;
while (root) {
preorder.push_back(root->val);
st.push(root);
root = root->left;
}
while (!st.empty()) {
root = st.top()->right;
st.pop();
while (root) {
preorder.push_back(root->val);
st.push(root);
root = root->left;
}
}
return preorder;
}
};