★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★
➤微信公众号：山青咏芝（shanqingyongzhi）
➤博客园地址：山青咏芝（https://www.cnblogs.com/strengthen/）
➤GitHub地址：https://github.com/strengthen/LeetCode
➤原文地址：https://www.cnblogs.com/strengthen/p/9951174.html
➤如果链接不是山青咏芝的博客园地址，则可能是爬取作者的文章。
➤原文已修改更新！强烈建议点击原文地址阅读！支持作者！支持原创！
★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★
Given a singly linked list where elements are sorted in ascending order, convert it to a height balanced BST.
For this problem, a height-balanced binary tree is defined as a binary tree in which the depth of the two subtrees of every node never differ by more than 1.
Example:
Given the sorted linked list: [-10,-3,0,5,9],
One possible answer is: [0,-3,9,-10,null,5], which represents the following height balanced BST:
0
/
-3 9
/ /
-10 5
给定一个单链表，其中的元素按升序排序，将其转换为高度平衡的二叉搜索树。
本题中，一个高度平衡二叉树是指一个二叉树每个节点 的左右两个子树的高度差的绝对值不超过 1。
示例:
给定的有序链表： [-10, -3, 0, 5, 9],
一个可能的答案是：[0, -3, 9, -10, null, 5], 它可以表示下面这个高度平衡二叉搜索树：
0
/
-3 9
/ /
-10 5
60ms
1 /** 2 * Definition for singly-linked list. 3 * public class ListNode { 4 * public var val: Int 5 * public var next: ListNode? 6 * public init(_ val: Int) { 7 * self.val = val 8 * self.next = nil 9 * } 10 * } 11 */ 12 /** 13 * Definition for a binary tree node. 14 * public class TreeNode { 15 * public var val: Int 16 * public var left: TreeNode? 17 * public var right: TreeNode? 18 * public init(_ val: Int) { 19 * self.val = val 20 * self.left = nil 21 * self.right = nil 22 * } 23 * } 24 */ 25 class Solution { 26 func sortedListToBST(_ head: ListNode?) -> TreeNode? { 27 // Count the length 28 var l = 0 29 var p = head 30 while p != nil { 31 l += 1 32 p = p?.next 33 } 34 35 // Build an empty height balanced binary tree 36 func buildTree(_ numNodes: Int) -> TreeNode? { 37 if numNodes == 0 { 38 return nil 39 } 40 let ret = TreeNode(0) 41 let numLeft = (numNodes - 1) / 2 42 let numRight = numNodes - 1 - numLeft 43 ret.left = buildTree(numLeft) 44 ret.right = buildTree(numRight) 45 return ret 46 } 47 let root = buildTree(l) 48 49 // Fill the tree 50 p = head 51 func dfs(_ n: TreeNode?) { 52 guard let node = n else { return } 53 dfs(node.left) 54 node.val = p!.val 55 p = p?.next 56 dfs(node.right) 57 } 58 59 dfs(root) 60 return root 61 } 62 }
64ms
1 /** 2 * Definition for singly-linked list. 3 * public class ListNode { 4 * public var val: Int 5 * public var next: ListNode? 6 * public init(_ val: Int) { 7 * self.val = val 8 * self.next = nil 9 * } 10 * } 11 */ 12 /** 13 * Definition for a binary tree node. 14 * public class TreeNode { 15 * public var val: Int 16 * public var left: TreeNode? 17 * public var right: TreeNode? 18 * public init(_ val: Int) { 19 * self.val = val 20 * self.left = nil 21 * self.right = nil 22 * } 23 * } 24 */ 25 class Solution { 26 func sortedListToBST(_ head: ListNode?) -> TreeNode? { 27 guard head != nil else { return nil } 28 var arr: [Int] = [] 29 var node = head 30 while node != nil { 31 arr.append(node!.val) 32 node = node?.next 33 } 34 let mid = arr.count/2 35 let root = TreeNode(arr[mid]) 36 appendBST(root, arr) 37 return root 38 } 39 40 private func appendBST(_ root: TreeNode?, _ arr: [Int]) { 41 guard arr.count > 1, let root = root else { return } 42 let mid = arr.count/2 43 let leftArr = Array(arr[0..<mid]) 44 let rightArr = Array(arr[mid+1..<arr.count]) 45 if leftArr.count > 1 { 46 root.left = TreeNode(leftArr[leftArr.count/2]) 47 appendBST(root.left, leftArr) 48 } else if leftArr.count == 1 { 49 root.left = TreeNode(leftArr[0]) 50 } 51 if rightArr.count > 1 { 52 root.right = TreeNode(rightArr[rightArr.count/2]) 53 appendBST(root.right, rightArr) 54 } else if rightArr.count == 1 { 55 root.right = TreeNode(rightArr[0]) 56 } 57 } 58 }
76ms
1 /** 2 * Definition for singly-linked list. 3 * public class ListNode { 4 * public var val: Int 5 * public var next: ListNode? 6 * public init(_ val: Int) { 7 * self.val = val 8 * self.next = nil 9 * } 10 * } 11 */ 12 /** 13 * Definition for a binary tree node. 14 * public class TreeNode { 15 * public var val: Int 16 * public var left: TreeNode? 17 * public var right: TreeNode? 18 * public init(_ val: Int) { 19 * self.val = val 20 * self.left = nil 21 * self.right = nil 22 * } 23 * } 24 */ 25 class Solution { 26 func sortedListToBST(_ head: ListNode?) -> TreeNode? { 27 var head = head 28 if head == nil { 29 return nil 30 } 31 let len = getLength(head) 32 var h = head 33 return sortedListToBST(0, len - 1, &head) 34 } 35 private func getLength(_ head: ListNode?) -> Int { 36 var head = head 37 var len = 0 38 while head != nil { 39 len += 1 40 head = head!.next 41 } 42 return len 43 } 44 private func sortedListToBST(_ start: Int, _ end: Int, _ currNode: inout ListNode?) -> TreeNode? { 45 if start > end { 46 return nil 47 } 48 let mid = (start + end) / 2 49 let left = sortedListToBST(start, mid - 1, &currNode) 50 var root = TreeNode(currNode!.val) 51 currNode = currNode!.next 52 let right = sortedListToBST(mid + 1, end, &currNode) 53 root.left = left 54 root.right = right 55 return root 56 } 57 }
88ms
1 /** 2 * Definition for singly-linked list. 3 * public class ListNode { 4 * public var val: Int 5 * public var next: ListNode? 6 * public init(_ val: Int) { 7 * self.val = val 8 * self.next = nil 9 * } 10 * } 11 */ 12 /** 13 * Definition for a binary tree node. 14 * public class TreeNode { 15 * public var val: Int 16 * public var left: TreeNode? 17 * public var right: TreeNode? 18 * public init(_ val: Int) { 19 * self.val = val 20 * self.left = nil 21 * self.right = nil 22 * } 23 * } 24 */ 25 class Solution { 26 func sortedListToBST(_ head: ListNode?) -> TreeNode? { 27 if head == nil { 28 return nil 29 } 30 if head?.next == nil{ 31 return TreeNode(head!.val) 32 } 33 var fast = head 34 var slow = head 35 var middle = head 36 while fast?.next != nil && fast?.next?.next != nil { 37 middle = slow 38 slow = slow?.next 39 fast = fast?.next?.next 40 } 41 fast = slow?.next 42 let root = TreeNode(slow!.val) 43 if head!.val != slow!.val { 44 middle?.next = nil 45 root.left = sortedListToBST(head) 46 } 47 root.right = sortedListToBST(fast) 48 return root 49 } 50 }
148ms
1 /** 2 * Definition for singly-linked list. 3 * public class ListNode { 4 * public var val: Int 5 * public var next: ListNode? 6 * public init(_ val: Int) { 7 * self.val = val 8 * self.next = nil 9 * } 10 * } 11 */ 12 /** 13 * Definition for a binary tree node. 14 * public class TreeNode { 15 * public var val: Int 16 * public var left: TreeNode? 17 * public var right: TreeNode? 18 * public init(_ val: Int) { 19 * self.val = val 20 * self.left = nil 21 * self.right = nil 22 * } 23 * } 24 */ 25 class Solution { 26 func sortedListToBST(_ head: ListNode?) -> TreeNode? { 27 var head = head 28 var array: [Int] = [] 29 while head != nil { 30 array.append(head!.val) 31 head = head?.next 32 } 33 34 return sortedArrayToBST(array) 35 } 36 37 func sortedArrayToBST(_ nums: [Int]) -> TreeNode? { 38 guard !nums.isEmpty else { 39 return nil 40 } 41 42 let rootIndex = nums.count/2 43 let root = TreeNode(nums[rootIndex]) 44 root.left = sortedArrayToBST([Int](nums[..<rootIndex])) 45 root.right = sortedArrayToBST([Int](nums[(rootIndex+1)...])) 46 47 return root 48 } 49 }