Kth Smallest Element in a BST

 1 /**
 2  * Definition for a binary tree node.
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class Solution {
11 public:
12     int kthSmallest(TreeNode* root, int k) {
13         stack <TreeNode*> tmp_s;
14         TreeNode* tmp=root;
15         while(tmp!=NULL){
16             tmp_s.push(tmp);
17             tmp=tmp->left;
18         }
19         while(k>0&&(tmp!=NULL||!tmp_s.empty())){
20             if(tmp==NULL){
21                 tmp=tmp_s.top();
22                 tmp_s.pop();
23                 if(--k==0)
24                     return tmp->val;
25                 tmp=tmp->right;
26             }
27             else{
28                 while(tmp!=NULL){
29                     tmp_s.push(tmp);
30                     tmp=tmp->left;
31                 }
32             }
33         }
34         return tmp->val;
35     }
36 };