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Given a binary tree, determine if it is a complete binary tree.
Definition of a complete binary tree from Wikipedia:
In a complete binary tree every level, except possibly the last, is completely filled, and all nodes in the last level are as far left as possible. It can have between 1 and 2h nodes inclusive at the last level h.
Example 1:
Input: [1,2,3,4,5,6]
Output: true
Explanation: Every level before the last is full (ie. levels with node-values {1} and {2, 3}), and all nodes in the last level ({4, 5, 6}) are as far left as possible.
Example 2:
Input: [1,2,3,4,5,null,7]
Output: false
Explanation: The node with value 7 isn't as far left as possible.
Note:
- The tree will have between 1 and 100 nodes.
给定一个二叉树，确定它是否是一个完全二叉树。
百度百科中对完全二叉树的定义如下：
若设二叉树的深度为 h，除第 h 层外，其它各层 (1～h-1) 的结点数都达到最大个数，第 h 层所有的结点都连续集中在最左边，这就是完全二叉树。（注：第 h 层可能包含 1~ 2h 个节点。）
示例 1：
输入：[1,2,3,4,5,6]
输出：true
解释：最后一层前的每一层都是满的（即，结点值为 {1} 和 {2,3} 的两层），且最后一层中的所有结点（{4,5,6}）都尽可能地向左。
示例 2：
输入：[1,2,3,4,5,null,7] 输出：false 解释：值为 7 的结点没有尽可能靠向左侧。
提示：
- 树中将会有 1 到 100 个结点。
20ms
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 isCompleteTree(_ root: TreeNode?) -> Bool { 16 var qs = [root] 17 while !qs.isEmpty { 18 let v = qs.count 19 for i in 0 ..< v { 20 guard let u = qs.remove(at: 0) else { 21 continue 22 } 23 var child = [u.left, u.right] 24 for c in child { 25 guard let t = c else { 26 qs.append(c) 27 continue 28 } 29 if qs.count > 0, let t = qs.last, t == nil { 30 return false 31 } 32 qs.append(c) 33 } 34 35 } 36 } 37 return true 38 } 39 }
36 ms
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 isCompleteTree(_ root: TreeNode?) -> Bool { 16 var root = root 17 var queue:[TreeNode?] = [TreeNode?]() 18 var leaf:Bool = false 19 queue.append(root) 20 21 while(!queue.isEmpty) 22 { 23 root = queue.removeFirst() 24 if (leaf && (root?.left != nil || root?.right != nil)) || (root?.left == nil && root?.right != nil) 25 { 26 return false 27 } 28 if root?.left != nil 29 { 30 queue.append(root?.left) 31 } 32 if root?.right != nil 33 { 34 queue.append(root?.right) 35 } 36 else 37 { 38 leaf = true 39 } 40 } 41 return true 42 } 43 }
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 extension TreeNode { 15 public func height() -> Int { 16 if self.left == nil && self.right == nil { 17 return 1 18 } 19 var left = 0, right = 0 20 if self.left != nil { 21 left = self.left!.height() 22 } 23 if self.right != nil { 24 right = self.right!.height() 25 } 26 return 1 + max(left, right) 27 } 28 } 29 30 class Solution { 31 func isCompleteTree(_ root: TreeNode?) -> Bool { 32 guard let first = root else { return true } 33 var level = first.height() 34 var arr: [TreeNode?] = [first] 35 var newArr: [TreeNode?] = [] 36 // Construct array 37 while level > 2 { 38 newArr = [] 39 for node in arr { 40 if node != nil && node?.left != nil && node?.right != nil { 41 newArr.append(node?.left) 42 newArr.append(node?.right) 43 } else { 44 return false 45 } 46 } 47 arr = newArr 48 level -= 1 49 } 50 51 52 // Assess last row 53 var foundNil = false 54 for node in arr { 55 // 1) If right node exists and left doesnt, return false 56 if node?.right != nil && node?.left == nil { 57 return false 58 } 59 // 2) If a nil was found, no subsequent node children should have values else return false 60 if foundNil && (node?.left != nil || node?.right != nil) { 61 return false 62 } 63 // 3) Notify a nil was found 64 if node?.left == nil || node?.right == nil { 65 foundNil = true 66 } 67 } 68 return true 69 } 70 }
40ms
1 extension TreeNode { 2 func elements() -> [Int?] { 3 let result: [Int?] = [self.val] 4 return elementsRec(queue: [self], result: result) 5 } 6 7 func elementsRec(queue: [TreeNode], result: [Int?]) -> [Int?] { 8 var innerQueue = queue 9 10 if innerQueue.isEmpty { 11 return result 12 } 13 14 let tree = innerQueue.removeFirst() 15 16 var innerResult = result 17 18 if tree.left == nil && tree.right == nil { 19 innerResult.append(nil) 20 innerResult.append(nil) 21 return elementsRec(queue: innerQueue, result: innerResult) 22 } 23 24 if let left = tree.left { 25 innerQueue.append(left) 26 innerResult.append(left.val) 27 } else { 28 innerResult.append(nil) 29 } 30 31 if let right = tree.right { 32 innerQueue.append(right) 33 innerResult.append(right.val) 34 } else { 35 innerResult.append(nil) 36 } 37 38 return elementsRec(queue: innerQueue, result: innerResult) 39 } 40 } 41 42 class Solution { 43 func isCompleteTree(_ root: TreeNode?) -> Bool { 44 guard let root = root else { 45 return true 46 } 47 48 let elements = root.elements() 49 50 var hasNull = false 51 52 for i in 0..<elements.count { 53 let element = elements[i] 54 55 if element == nil { 56 57 hasNull = true 58 continue 59 60 } else { 61 62 if hasNull { 63 return false 64 } 65 66 hasNull = false 67 } 68 } 69 70 if root.left == nil && root.right != nil { 71 return false 72 } 73 74 return isCompleteTree(root.left) && isCompleteTree(root.right) 75 } 76 }