230. Kth Smallest Element in a BST

 

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。

class Solution {
    public int kthSmallest(TreeNode root, int k) {
        if(root==null) return 0;
        TreeNode cur = root;
        Stack<TreeNode> stack = new Stack<TreeNode>();
        while(cur!=null ||!stack.isEmpty()){
            while(cur!= null){
            stack.push(cur);
            cur = cur.left;
            }
            cur = stack.pop();
            k--;
            if(k==0) return cur.val;
            cur = cur.right;
        }
        return 0;
    }
}