https://leetcode.com/problems/kth-smallest-element-in-a-bst/

1. Use inorder traverse:

  1. //20 ms    24.3 MB
  3. /**
  4.  * Definition for a binary tree node.
  5.  * struct TreeNode {
  6.  *     int val;
  7.  *     TreeNode *left;
  8.  *     TreeNode *right;
  9.  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
  10.  * };
  11.  */
  12. class Solution {
  13. public:
  14.     int kthSmallest(TreeNode* root, int k) {
  15.         return inorder(root, k);
  16.     }
  18. private: 
  19.     int inorder(TreeNode* root, int& k) {
  20.         if(!root) return -1;
  22.         int x = inorder(root->left, k);
  24.         if (k == 0) return x;
  26.         k--;
  27.         if (k == 0) return root->val;
  29.         return inorder(root->right, k);
  30.     }
  31. };