[Swift]LeetCode1008. 先序遍歷構造二叉樹 | Construct Binary Search Tree from Preorder Traversal

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Return the root node of a binary search tree that matches the given preorder traversal.

(Recall that a binary search tree is a binary tree where for every node, any descendant of node.left has a value < node.val, and any descendant of node.right has a value > node.val.  Also recall that a preorder traversal displays the value of the node first, then traverses node.left, then traverses node.right.) 

Example 1:

Input: [8,5,1,7,10,12]
Output: [8,5,10,1,7,null,12] 

Note: 

1. 1 <= preorder.length <= 100
2. The values of preorder are distinct.

---

返回與給定先序遍歷 preorder 相匹配的二叉搜索樹（binary search tree）的根結點。

(回想一下，二叉搜索樹是二叉樹的一種，其每一個節點都知足如下規則，對於 node.left 的任何後代，值總 < node.val，而 node.right 的任何後代，值總 > node.val。此外，先序遍歷首先顯示節點的值，而後遍歷 node.left，接着遍歷 node.right。）

示例：

輸入：[8,5,1,7,10,12]
輸出：[8,5,10,1,7,null,12]

提示：

1. 1 <= preorder.length <= 100
2. 先序 preorder 中的值是不一樣的。

---

 8ms

 1 /**
 2  * Definition for a binary tree node.
 3  * public class TreeNode {
 4  *     public var val: Int
 5  *     public var left: TreeNode?
 6  *     public var right: TreeNode?
 7  *     public init(_ val: Int) {
 8  *         self.val = val
 9  *         self.left = nil
10  *         self.right = nil
11  *     }
12  * }
13  */
14 class Solution {
15     var i = 0
16     
17     func bstFromPreorder(_ preorder: [Int]) -> TreeNode? {
18         return bstFromPreorder(preorder, Int.max)
19     }
20     
21     func bstFromPreorder(_ preorder: [Int], _ bound: Int) -> TreeNode? {
22         guard i < preorder.count, preorder[i] < bound else {
23             return nil
24         }
25         
26         var root = TreeNode(preorder[i])
27         i += 1
28         root.left = bstFromPreorder(preorder, root.val)
29         root.right = bstFromPreorder(preorder, bound)
30         return root
31     }
32 }

---

Runtime: 12 ms

Memory Usage: 18.8 MB

 1 /**
 2  * Definition for a binary tree node.
 3  * public class TreeNode {
 4  *     public var val: Int
 5  *     public var left: TreeNode?
 6  *     public var right: TreeNode?
 7  *     public init(_ val: Int) {
 8  *         self.val = val
 9  *         self.left = nil
10  *         self.right = nil
11  *     }
12  * }
13  */
14 class Solution {
15     func bstFromPreorder(_ preorder: [Int]) -> TreeNode? {
16         var n:Int = preorder.count
17         if n == 0 {return nil}
18         var val:Int = preorder[0]
19         var root:TreeNode? = TreeNode(val)
20         var left:[Int] = [Int]()
21         var right:[Int] = [Int]()
22         for i in 1..<n
23         {
24             if preorder[i] < val
25             {
26                 left.append(preorder[i])
27             }
28             else
29             {
30                 right.append(preorder[i])
31             }
32         }
33         root?.left = bstFromPreorder(left)
34         root?.right = bstFromPreorder(right)
35         return root
36     }
37 }

---

12ms

 

 1 class Solution {
 2     func bstFromPreorder(_ preorder: [Int]) -> TreeNode? {
 3         return trialOne(a: preorder)
 4     }
 5     
 6     func trialOne(a: [Int]) -> TreeNode {
 7         guard a.count > 1 else { return TreeNode(a[0]) }
 8         let root = TreeNode(a[0])
 9         let largerIndex = findElementLarger(start: 1, end: a.count-1, a: a) ?? -1
10         if largerIndex != -1 {
11             root.right = trialOne(a: Array(a[largerIndex...a.count-1]))
12         }
13         guard let smallerIndex = findElementSmaller(start: 1, end: a.count-1, a: a) else { return root }
14         if largerIndex != -1 {
15             root.left = trialOne(a: Array(a[smallerIndex...largerIndex-1]))
16         } else {
17             root.left = trialOne(a: Array(a[smallerIndex...a.count-1]))
18         }
19         return root
20     }
21     
22     func findElementLarger(start: Int, end: Int, a: [Int]) -> Int? {
23         guard a.count > 1 else { return nil }
24         for i in start...end {
25             if a[i] > a[0] {
26                 return i
27             }
28         }
29         return nil
30     }
31     
32     func findElementSmaller(start: Int, end: Int, a: [Int]) -> Int? {
33         guard a.count > 1 else { return nil }
34         for i in start...end {
35             if a[i] < a[0] {
36                 return i
37             }
38         }
39         return nil
40     }
41 }

---

16ms

 1 class Solution {
 2     func bstFromPreorder(_ preorder: [Int]) -> TreeNode? {
 3         return buildTree(0, preorder.count-1, preorder)
 4     }
 5 
 6     func buildTree(_ start: Int, _ end: Int, _ preorder: [Int]) -> TreeNode? {
 7         if start > end { return nil }     
 8         if start == end { return TreeNode(preorder[start]) }
 9         let node = TreeNode(preorder[start])
10         var index = start
11         for i in start+1 ... end {
12             if preorder[i] > node.val {
13                 index = i
14                 break
15             }
16         }
17 
18         if index == start {
19             node.left = buildTree(start+1, end, preorder)
20         }
21         else {
22             node.left = buildTree(start+1, index-1, preorder)
23             node.right = buildTree(index, end, preorder)
24         }
25         return node
26     }
27 }