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Two elements of a binary search tree (BST) are swapped by mistake.
Recover the tree without changing its structure.
Example 1:
Input: [1,3,null,null,2] 1 / 3 2 Output: [3,1,null,null,2] 3 / 1 2
Example 2:
Input: [3,1,4,null,null,2] 3 / 1 4 / 2 Output: [2,1,4,null,null,3] 2 / 1 4 / 3
Follow up:
- A solution using O(n) space is pretty straight forward.
- Could you devise a constant space solution?
二叉搜索树中的两个节点被错误地交换。
请在不改变其结构的情况下，恢复这棵树。
示例 1:
输入: [1,3,null,null,2] 1 / 3 2 输出: [3,1,null,null,2] 3 / 1 2
示例 2:
输入: [3,1,4,null,null,2] 3 / 1 4 / 2 输出: [2,1,4,null,null,3] 2 / 1 4 / 3
进阶:
- 使用 O(n) 空间复杂度的解法很容易实现。
- 你能想出一个只使用常数空间的解决方案吗？
68ms
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 biggerNode: TreeNode? = nil 16 var smallerNode: TreeNode? = nil 17 var node = TreeNode(-10000) 18 19 func inorderTraverse(_ root: TreeNode) { 20 21 if root.left != nil { 22 inorderTraverse(root.left!) 23 } 24 25 if biggerNode == nil && root.val <= node.val { 26 biggerNode = node 27 } 28 if root.val <= node.val { 29 smallerNode = root 30 } 31 node = root 32 33 if root.right != nil { 34 inorderTraverse(root.right!) 35 } 36 37 } 38 func recoverTree(_ root: TreeNode?) { 39 guard let root = root else { return } 40 inorderTraverse(root) 41 let num = smallerNode!.val 42 smallerNode?.val = (biggerNode?.val)! 43 biggerNode?.val = num 44 } 45 }
88ms
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 recoverTree(_ root: TreeNode?) { 16 var firstElement: TreeNode? = nil 17 var secondElement: TreeNode? = nil 18 var prevElement = TreeNode(Int.min) 19 20 traverse(root, &firstElement, &secondElement, &prevElement) 21 22 let temp = firstElement?.val 23 firstElement?.val = secondElement?.val ?? 0 24 secondElement?.val = temp ?? 0 25 } 26 27 fileprivate func traverse(_ root: TreeNode?, _ firstElement: inout TreeNode?, _ secondElement: inout TreeNode?, _ prevElement: inout TreeNode) { 28 guard let root = root else { return } 29 30 traverse(root.left, &firstElement, &secondElement, &prevElement) 31 32 if firstElement == nil && prevElement.val >= root.val { 33 firstElement = prevElement 34 } 35 if firstElement != nil && prevElement.val >= root.val { 36 secondElement = root 37 } 38 prevElement = root 39 40 traverse(root.right, &firstElement, &secondElement, &prevElement) 41 } 42 }
100ms
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 first: TreeNode? 16 var second: TreeNode? 17 var prev = TreeNode(Int.min) 18 19 func recoverTree(_ root: TreeNode?) { 20 traverse(root) 21 22 let temp = first!.val 23 first!.val = second!.val 24 second!.val = temp 25 } 26 27 func traverse(_ root: TreeNode?) { 28 if root == nil { 29 return 30 } 31 32 traverse(root?.left) 33 34 if first == nil && prev.val >= root!.val { 35 first = prev 36 } 37 38 if first != nil && prev.val >= root!.val { 39 second = root 40 } 41 prev = root! 42 43 traverse(root?.right) 44 } 45 }
128ms
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 recoverTree(_ root: TreeNode?) { 16 guard root != nil else { 17 return 18 } 19 20 var root = root 21 22 var firstNode: TreeNode? 23 var secondNode: TreeNode? 24 var preNode: TreeNode? 25 26 while root != nil { 27 if root!.left != nil { 28 var pre = root!.left 29 while pre!.right != nil && root! !== pre!.right! { 30 pre = pre!.right 31 } 32 33 if pre!.right != nil && root === pre!.right! { 34 if preNode != nil && preNode!.val >= root!.val { 35 if firstNode == nil { 36 firstNode = preNode 37 } 38 secondNode = root 39 } 40 preNode = root 41 42 root = root!.right 43 pre!.right = nil 44 } else { 45 pre!.right = root 46 root = root!.left 47 } 48 49 } else { 50 51 if preNode != nil && preNode!.val >= root!.val { 52 if firstNode == nil { 53 firstNode = preNode 54 } 55 secondNode = root 56 } 57 preNode = root 58 59 root = root!.right 60 } 61 } 62 63 if firstNode != nil && secondNode != nil { 64 let temp = firstNode!.val 65 firstNode!.val = secondNode!.val 66 secondNode!.val = temp 67 } 68 } 69 }