230. Kth Smallest Element in a BST

Given a binary search tree, write a function

kthSmallest

to find thekth 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?

Hint:

Try to utilize the property of a BST.
What if you could modify the BST node's structure?
The optimal runtime complexity is O(height of BST).

思路：

二叉搜索树左节点小于根，右节点大于根，使用中序遍历

/**
* Definition for a binary tree node.
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public int kthSmallest(TreeNode root, int k) {
Stack<TreeNode> stack = new Stack<>();
TreeNode p = root;
while (k > 0 && (p != null || !stack.empty())) {
while(p != null){
stack.push(p);
p = p.left;
}
p = stack.peek();
stack.pop();
k--;
if(k == 0)
return p.val;
p = p.right;
}
return p.val;
}
}