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230.Kth-Smallest-Element-in-a-BST

230. Kth Smallest Element in a BST

题目地址

题目描述

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.
Example 1:
Input: root = [3,1,4,null,2], k = 1
3
/ \
1 4
\
2
Output: 1
Example 2:
Input: root = [5,3,6,2,4,null,null,1], k = 3
5
/ \
3 6
/ \
2 4
/
1
Output: 3
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?

代码

Approach #1 Recursion

/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Solution {
public int kthSmallest(TreeNode root, int k) {
ArrayList<Integer> nums = inorder(root, new ArrayList<Integer>());
return nums.get(k - 1);
}
public ArrayList<Integer> inorder(TreeNode root, ArrayList<Integer> arr) {
if (root == null) return arr;
inorder(root.left, arr);
arr.add(root.val);
inorder(root.right, arr);
return arr;
}
}

Approach #2 Iteration

class Solution {
public int kthSmallest(TreeNode root, int k) {
LinkedList<TreeNode> stack = new LinkedList<TreeNode>();
while (true) {
while (root != null) {
stack.add(root);
root = root.left;
}
root = stack.removeLast();
if (--k == 0) return root.val;
root = root.right;
}
}
}