230. Kth Smallest Element in a BST

Posted 2020-10-22 张乐乐章

 

Given a binary search tree, write a function kthSmallest to find the kth smallest element in it.

Note: 
You may assume k is always valid, 1 ≤ k ≤ BST‘s total elements.

Follow up:
What if the BST is modified (insert/delete operations) often and you need to find the kth smallest frequently? How would you optimize the kthSmallest routine?

Credits:
Special thanks to @ts for adding this problem and creating all test cases.

 

利用中序遍历， 与前面2个题思路一样。

 

 

 1 class Solution {
 2     public int kthSmallest(TreeNode root, int k) {
 3         if(root==null) return 0;
 4         TreeNode cur = root;
 5         Stack<TreeNode> stack = new Stack<TreeNode>();
 6         while(cur!=null ||!stack.isEmpty()){
 7             while(cur!= null){
 8             stack.push(cur);
 9             cur = cur.left;
10             }
11             cur = stack.pop();
12             k--;
13             if(k==0) return cur.val;
14             cur = cur.right;
15         }
16         return 0;
17     }
18 }