LeetCode 230 Kth Smallest Element in a BST

题目描述

Given a binary search tree, write a function kth Smallest 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?

Hint:

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

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

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分析

因为是BST，那么中序遍历即得到从小到大的序列。中序遍历，到第k次即可。

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代码

    public int kthSmallest(TreeNode root, int k) {        if (root == null) {            return -1;        }        Stack<TreeNode> stack = new Stack<TreeNode>();        TreeNode p = root;        while (p != null || !stack.isEmpty()) {            while (p != null) {                stack.push(p);                p = p.left;            }            if (!stack.isEmpty()) {                p = stack.pop();                if (--k == 0) {                    return p.val;                }                p = p.right;            }        }        return 0;    }