0x00 topic
Given the root node of a binary search tree root
and an integer k
, please design an algorithm to find k
the smallest element in it ( 1
counting from )
0x01 Ideas
Method 1:
中序
Traverse the binary search tree
and 数组
save when the last element of the
array is 数量
equal to the smallestk
k
Method 2:
中序
Traverse the binary search tree, which is incremental.
Use a marker to record k
the first recursion, k
reduce to 1
when it isk
, find the target0
0x02 solution
language:Swift
tree node:TreeNode
public class TreeNode {
public var val: Int
public var left: TreeNode?
public var right: TreeNode?
public init() { self.val = 0; self.left = nil; self.right = nil; }
public init(_ val: Int) { self.val = val; self.left = nil; self.right = nil; }
public init(_ val: Int, _ left: TreeNode?, _ right: TreeNode?) {
self.val = val
self.left = left
self.right = right
}
}
solution:
func kthSmallest(_ root: TreeNode?, _ k: Int) -> Int {
var res = Int.min
var i = k
func dfs(_ root: TreeNode?) {
guard let r = root else { return }
if i == 0 {
return
}
// 前序遍历位置
dfs(r.left)
// 中序遍历位置
i -= 1
if i == 0 {
res = r.val
}
dfs(r.right)
// 后序遍历位置
}
dfs(root)
return res
}