144. Binary Tree Preorder Traversal #Medium

/**
* 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 inorderTraversal(TreeNode* root) {
vector r;
stack s;
for(auto p=root;p || !s.empty();){
for(;p;p=p->left)s.push(p);
p=s.top();
r.push_back(p->val);
s.pop();
p=p->right;
}
return r;
}
};