Given preorder and inorder traversal of a tree, construct the binary tree.
Note:
You may assume that duplicates do not exist in the tree.
For example, given
preorder = [3,9,20,15,7] inorder = [9,3,15,20,7]
Return the following binary tree:
3
/ 9 20
/ 15 7
根据一棵树的前序遍历与中序遍历构造二叉树。
注意:
你可以假设树中没有重复的元素。
例如，给出
前序遍历 preorder = [3,9,20,15,7] 中序遍历 inorder = [9,3,15,20,7]
返回如下的二叉树：
3
/ 9 20
/ 15 7
32ms
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
15 extension Array {
16 subscript(safe index: Int) -> Element? {
17 guard index >= 0 && index < self.count else {
18 return nil
19 }
20
21 return self[index]
22 }
23 }
24
25 class Solution {
26 func buildTree(_ preorder: [Int], _ inorder: [Int]) -> TreeNode? {
27 guard let rootValue = preorder[safe: 0] else {
28 return nil
29 }
30
31 var inorderMap: [Int: Int] = [:]
32 for index in 0..<inorder.count {
33 inorderMap[inorder[index]] = index
34 }
35 let root = TreeNode(rootValue)
36
37 buildNextNode(inorder: inorder, preorder: preorder, inorderMap: inorderMap, currentPreOrderIndex: 0, root: root, tooFarValue: nil)
38
39 return root
40 }
41
42 func buildNextNode(inorder: [Int], preorder: [Int], inorderMap: [Int: Int], currentPreOrderIndex: Int, root: TreeNode, tooFarValue: Int?) -> Int {
43 var currentPreOrderIndex = currentPreOrderIndex
44 guard let nextPreOrderValue = preorder[safe: currentPreOrderIndex + 1] else {
45 return currentPreOrderIndex
46 }
47
48 let childDirection = whichChildDirection(inorder: inorder, inorderMap: inorderMap, knownValue: root.val, potentialChildValue: nextPreOrderValue, tooFarValue: tooFarValue)
49
50 guard let direction = childDirection else {
51 // not a child
52 return currentPreOrderIndex
53 }
54
55 let childNode = TreeNode(nextPreOrderValue)
56
57 if direction {
58 // right child
59 root.right = childNode
60 return buildNextNode(inorder: inorder, preorder: preorder, inorderMap: inorderMap, currentPreOrderIndex: currentPreOrderIndex + 1, root: childNode, tooFarValue: tooFarValue)
61
62 } else {
63 // left child
64 root.left = childNode
65 currentPreOrderIndex = buildNextNode(inorder: inorder, preorder: preorder, inorderMap: inorderMap, currentPreOrderIndex: currentPreOrderIndex + 1, root: childNode, tooFarValue: root.val)
66 return buildNextNode(inorder: inorder, preorder: preorder, inorderMap: inorderMap, currentPreOrderIndex: currentPreOrderIndex, root: root, tooFarValue: tooFarValue)
67 }
68
69 return currentPreOrderIndex
70 }
71
72 /**
73 false: left child
74 true: right child
75 nil: neither child
76 */
77 func whichChildDirection(inorder: [Int], inorderMap: [Int: Int], knownValue: Int, potentialChildValue: Int, tooFarValue: Int?) -> Bool? {
78 let potentialChildIndex = inorderMap[potentialChildValue]! // guarenteed to be in the map
79 let knownIndex = inorderMap[knownValue]! // guarenteed to be in the map
80
81 if potentialChildIndex < knownIndex {
82 return false
83 }
84
85 if let tooFarValue = tooFarValue, let tooFarIndex = inorderMap[tooFarValue] {
86 if tooFarIndex < potentialChildIndex {
87 return nil
88 }
89 }
90
91 return true
92 }
93 }
36ms
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 buildTree(_ preorder: [Int], _ inorder: [Int]) -> TreeNode? {
16
17 guard preorder.count > 0, inorder.count > 0 else {
18 return nil
19 }
20
21 var inorderDict = [Int: Int]()
22 for (i, item) in inorder.enumerated() {
23 inorderDict[item] = i
24 }
25
26 return create(inorderDict, preorder, 0, preorder.count - 1, inorder, 0 , inorder.count - 1)
27
28 }
29
30 func create(_ inorderDict: [Int: Int], _ preorder: [Int], _ preLow: Int, _ preHi: Int, _ inorder: [Int], _ inLow: Int, _ inHi: Int) -> TreeNode? {
31
32 if preLow > preHi { //debug
33 return nil
34 }
35
36 var root = TreeNode(preorder[preLow])
37 let rootIndex = inorderDict[preorder[preLow]]! // debug: dict value is optional, must be unwrapped
38 let leftNum = rootIndex - inLow // debug
39
40 root.left = create(inorderDict, preorder, preLow + 1, preLow + leftNum, inorder, inLow, rootIndex - 1)
41 root.right = create(inorderDict, preorder, preLow + leftNum + 1, preHi, inorder, rootIndex + 1, inHi)
42
43 return root
44 }
45 }
44ms
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 buildTree(_ preorder: [Int], _ inorder: [Int]) -> TreeNode? {
16 if preorder.count == 0 {
17 return nil
18 }
19
20 var map = [Int: Int]()
21 for i in 0..<inorder.count {
22 map[inorder[i]] = i
23 }
24
25 var stack = [TreeNode]()
26 var value = preorder[0]
27 let root = TreeNode(value)
28
29 stack.append(root)
30 for i in 1..<preorder.count {
31 value = preorder[i]
32 let node = TreeNode(value)
33 if map[value]! < map[stack.last!.val]! {
34 stack.last!.left = node
35 } else {
36 var parent: TreeNode? = nil
37 while !stack.isEmpty && map[value]! > map[stack.last!.val]! {
38 parent = stack.removeLast()
39 }
40 parent?.right = node
41 }
42 stack.append(node)
43 }
44
45 return root
46 }
47 }
56ms
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 buildTree(_ preorder: [Int], _ inorder: [Int]) -> TreeNode? {
16
17 guard !preorder.isEmpty && !inorder.isEmpty else {
18 return nil
19 }
20
21 return buildTree(preorder, 0, preorder.count-1, inorder, 0, inorder.count-1)
22 }
23
24 func buildTree(_ preorder: [Int], _ preStart: Int, _ preEnd: Int, _ inorder: [Int], _ inStart: Int, _ inEnd: Int) -> TreeNode? {
25
26 guard preStart <= preEnd && inStart <= inEnd else {
27 return nil
28 }
29
30 var rootVal = preorder[preStart]
31 var root = TreeNode(rootVal)
32 var mid: Int = 0
33
34 for i in inStart...inEnd {
35 if inorder[i] == rootVal {
36 mid = i
37 break
38 }
39 }
40
41 root.left = buildTree(preorder, preStart+1, preStart + mid - inStart, inorder, inStart, mid - 1)
42 root.right = buildTree(preorder, preStart + mid - inStart + 1, preEnd, inorder, mid+1, inEnd)
43 return root
44 }
45 }
160ms
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 buildTree(_ preorder: [Int], _ inorder: [Int]) -> TreeNode? {
16 let len = preorder.count
17 if len == 0 { return nil }
18 let root = TreeNode(preorder[0])
19
20 var i = 0
21 while i < len {
22 if inorder[i] == root.val {
23 break
24 }
25 i += 1
26 }
27
28 if i > 0 {
29 root.left = buildTree(Array(preorder[1...i]), Array(inorder[..<i]))
30 }
31 if i < len-1 {
32 root.right = buildTree(Array(preorder[(i+1)...]), Array(inorder[(i+1)...]))
33 }
34
35 return root
36 }
37 }
240ms
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 // 根据中序和前序遍历构建二叉树
16 func buildTree(_ inorder: [Int], _ preorder: [Int]) -> TreeNode? {
17
18 if inorder.count <= 0 || preorder.count <= 0{
19 return nil
20 }
21 /*
22 * 后序遍历的最后一个节点肯定是整个树的根节点
23 */
24 return buildTrees(preorder, inorder)
25 }
26
27 func buildTrees(_ inorder: [Int], _ preorder: [Int]) -> TreeNode{
28
29 // 根据前序遍历的第一个节点作为根节点
30 let root = TreeNode.init(preorder[0])
31
32 // 根据中序遍历计算根节点左右子节点的个数
33 var indexRoot: Int = -1
34
35 for index in 0..<inorder.count{
36 if inorder[index] == preorder[0]{
37 indexRoot = index
38 break
39 }
40 }
41
42 // 左子节点的个数
43 let leftNum = indexRoot
44 // 右子节点的个数
45 let rightNum = inorder.count - indexRoot - 1
46
47 // 生成新的左右子树
48 if indexRoot > 0{
49 let leftNode = buildTrees([Int](inorder[0...indexRoot - 1]), [Int](preorder[1...leftNum]))
50 root.left = leftNode
51 }
52 if indexRoot < inorder.count - 1{
53 let rightNode = buildTrees([Int](inorder[indexRoot + 1...inorder.count - 1]), [Int](preorder[leftNum + 1...leftNum + rightNum]))
54 root.right = rightNode
55 }
56 return root
57 }
58 }
288ms
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 buildTree(_ preorder: [Int], _ inorder: [Int]) -> TreeNode? {
16
17 if preorder.count == 0 {
18 return nil
19 }
20
21 if preorder.count == 1 {
22 let val = preorder[0]
23 let root = TreeNode(val)
24 return root
25 }
26
27 let val = preorder[0]
28 let root = TreeNode(val)
29
30 var rootIndex = 0
31 for i in 0..<inorder.count {
32 if inorder[i] == val {
33 rootIndex = i
34 }
35 }
36
37 var tmpPreorder = [Int]()
38 if rootIndex > 0 {
39 for i in 1...rootIndex {
40 tmpPreorder.append(preorder[i])
41 }
42 }
43
44 var tmpInorder = [Int]()
45 for i in 0..<rootIndex {
46 tmpInorder.append(inorder[i])
47 }
48
49 root.left = buildTree(tmpPreorder, tmpInorder)
50
51 tmpPreorder = [Int]()
52 for i in rootIndex+1..<preorder.count {
53 tmpPreorder.append(preorder[i])
54 }
55
56 tmpInorder = [Int]()
57 for i in rootIndex+1..<inorder.count {
58 tmpInorder.append(inorder[i])
59 }
60
61 root.right = buildTree(tmpPreorder, tmpInorder)
62
63 return root
64 }
65 }
328ms
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 buildTree(_ preorder: [Int], _ inorder: [Int]) -> TreeNode? {
16 guard preorder.count != 0, inorder.count != 0, preorder.count == inorder.count else { return nil }
17 let root = TreeNode(preorder[0])
18 let rootInOrderIndex = inorder.index(of: root.val) ?? -1
19 let leftInOrder = Array(inorder[0..<rootInOrderIndex])
20 let leftPreOrderStartIndex = 1
21 let leftPreOrderEndIndexExclusive = leftPreOrderStartIndex + leftInOrder.count
22 let leftPreOrder = Array(preorder[leftPreOrderStartIndex..<leftPreOrderEndIndexExclusive])
23 root.left = buildTree(leftPreOrder, leftInOrder)
24
25 let rightInOrder = Array(inorder[rootInOrderIndex+1..<inorder.count])
26 let rightPreOrder = Array(preorder[leftPreOrderEndIndexExclusive..<preorder.count])
27 root.right = buildTree(rightPreOrder, rightInOrder)
28 return root
29 }
30 }
332ms
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 buildTree(_ preorder: [Int], _ inorder: [Int]) -> TreeNode? {
16 var inorder = inorder
17 var preorder = preorder
18 var root: TreeNode
19 if preorder.count < 1{
20 return nil
21 }
22 let first = preorder.removeFirst()
23 root = TreeNode(first)
24 let index = inorder.index(of: first)!
25 let lin = Array(inorder[0..<index])
26 let lpro = Array(preorder[0..<index])
27 root.left = buildTree(lpro, lin)
28 let rin = Array(inorder[(index+1)...])
29 let rpro = Array(preorder[index...])
30 root.right = buildTree(rpro, rin)
31 return root
32 }
33 }
原文地址：https://www.cnblogs.com/strengthen/p/9950714.html
时间： 2024-08-17 13:14:08