[Swift]LeetCode110. 平衡二叉树 | Balanced Binary Tree
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Given a binary tree, determine if it is height-balanced.
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 1:
Given the following tree [3,9,20,null,null,15,7]:
3
/ \
9 20
/ \
15 7
Return true.
Example 2:
Given the following tree [1,2,2,3,3,null,null,4,4]:
1
/ \
2 2
/ \
3 3
/ \
4 4
Return false.
给定一个二叉树，判断它是否是高度平衡的二叉树。
本题中，一棵高度平衡二叉树定义为：
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 1:
给定二叉树 [3,9,20,null,null,15,7]
3
/ \
9 20
/ \
15 7
返回 true 。
示例 2:
给定二叉树 [1,2,2,3,3,null,null,4,4]
1
/ \
2 2
/ \
3 3
/ \
4 4
返回 false 。
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 isBalanced(_ root: TreeNode?) -> Bool { 16 //递归分治法 17 if root == nil {return true} 18 19 var depthLeft:Int = depth(root?.left) 20 var depthRight:Int = depth(root?.right) 21 if abs(depthLeft-depthRight) > 1 22 { 23 return false 24 } 25 else 26 { 27 return isBalanced(root?.left) && isBalanced(root?.right) 28 } 29 } 30 //求树的高度 31 func depth(_ n:TreeNode?)->Int 32 { 33 if n==nil 34 { 35 return 0 36 } 37 else if n?.left == nil && n?.right == nil 38 { 39 return 1 40 } 41 else 42 { 43 var depthLeft:Int = depth(n?.left) 44 var depthRight:Int = depth(n?.right) 45 return (1 + ((depthLeft > depthRight) ? depthLeft : depthRight)) 46 } 47 } 48 }
48ms
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 16 // func isBalanced(_ root: TreeNode?) -> Bool { 17 // guard let root = root else { 18 // return true 19 // } 20 // if abs(getDepth(root.left) - getDepth(root.right)) > 1 { 21 // return false 22 // } 23 // return isBalanced(root.left) && isBalanced(root.right) 24 // } 25 26 // func getDepth(_ root: TreeNode?) -> Int { 27 // if root == nil { 28 // return 0 29 // } 30 // return max(getDepth(root!.left), getDepth(root!.right)) + 1 31 // } 32 33 func isBalanced(_ root: TreeNode?) -> Bool { 34 if checkDepth(root) == -1 { 35 return false 36 } 37 return true 38 } 39 40 func checkDepth(_ root: TreeNode?) -> Int { 41 guard let root = root else { 42 return 0 43 } 44 45 let left = checkDepth(root.left) 46 if left == -1 { 47 return -1 48 } 49 50 let right = checkDepth(root.right) 51 if right == -1 { 52 return -1 53 } 54 55 if abs(left - right) > 1 { 56 return -1 57 } 58 59 return max(left, right) + 1 60 } 61 }
52ms
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 isBalanced(_ root: TreeNode?) -> Bool { 16 var result = true 17 isBalanced(root, &result) 18 return result 19 } 20 21 func isBalanced(_ root: TreeNode?, _ result: inout Bool) -> Int { 22 guard let root = root else { 23 return 0 24 } 25 26 guard result else { 27 return 0 28 } 29 30 let leftHeight = isBalanced(root.left, &result) 31 let rightHeight = isBalanced(root.right, &result) 32 if abs(leftHeight - rightHeight) > 1 { 33 result = false 34 } 35 36 return 1 + max(leftHeight, rightHeight) 37 } 38 }
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 class Solution { 15 func isBalanced(_ root: TreeNode?) -> Bool { 16 return root?.depth() != Int.max 17 } 18 19 } 20 21 extension TreeNode { 22 func depth(withPreviousDepth depth : Int = 1) -> Int { 23 let leftDepth = self.left?.depth(withPreviousDepth: depth + 1) ?? depth 24 let rightDepth = self.right?.depth(withPreviousDepth: depth + 1) ?? depth 25 26 // looks repetitive, but it has better real-world 27 // runtime performance than using max() and abs() 28 if (leftDepth > rightDepth) { 29 guard leftDepth - rightDepth <= 1 else { return Int.max } 30 return leftDepth 31 } else { 32 guard rightDepth - leftDepth <= 1 else { return Int.max } 33 return rightDepth 34 } 35 } 36 }
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 isBalanced(_ root: TreeNode?) -> Bool { 16 return dfs(root) != -1 17 } 18 19 func dfs(_ node: TreeNode?) -> Int { 20 21 if node == nil { 22 return 0 23 } 24 25 let leftHeight = dfs(node?.left) 26 27 if leftHeight == -1 { 28 return -1 29 } 30 31 let rightHeight = dfs(node?.right) 32 33 if rightHeight == -1 { 34 return -1 35 } 36 37 let totalHeight = abs(leftHeight - rightHeight) 38 39 if totalHeight > 1 { 40 return -1 41 } 42 43 return max(leftHeight, rightHeight) + 1 44 } 45 46 }
