230. Kth Smallest Element in a BST

发表于 | |

230. Kth Smallest Element in a BST

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.

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?

题目大意

找二叉搜索树中，第 k 个小的值。

解题思路

通过辅助栈，按照中序遍历二叉树。

代码

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public var val: Int
 *     public var left: TreeNode?
 *     public var right: TreeNode?
 *     public init(_ val: Int) {
 *         self.val = val
 *         self.left = nil
 *         self.right = nil
 *     }
 * }
 */
class Solution {
    func kthSmallest(_ root: TreeNode?, _ k: Int) -> Int {
        var tree = root
        var stack = [TreeNode?]()
        while tree != nil {
            stack.append(tree)
            tree = tree?.left
        }
        var num = 1
        while !stack.isEmpty && num <= k {
            tree = stack.removeLast()
            num += 1
            var r = tree?.right
            while r != nil {
                stack.append(r)
                r = r?.left
            }
        }
        return tree?.val ?? 0
    }
}
0%