[Swift]LeetCode331. 验证二叉树的前序序列化 | Verify Preorder Serialization of a Binary Tree

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One way to serialize a binary tree is to use pre-order traversal. When we encounter a non-null node, we record the node's value. If it is a null node, we record using a sentinel value such as #.

     _9_
    /   \
   3     2
  / \   / \
 4   1  #  6
/ \ / \   / \
# # # #   # #

For example, the above binary tree can be serialized to the string "9,3,4,#,#,1,#,#,2,#,6,#,#", where # represents a null node.

Given a string of comma separated values, verify whether it is a correct preorder traversal serialization of a binary tree. Find an algorithm without reconstructing the tree.

Each comma separated value in the string must be either an integer or a character '#' representing null pointer.

You may assume that the input format is always valid, for example it could never contain two consecutive commas such as "1,,3".

Example 1:

Input: "9,3,4,#,#,1,#,#,2,#,6,#,#"
Output: true

Example 2:

Input: "1,#"
Output: false

Example 3:

Input: "9,#,#,1"
Output: false

---

序列化二叉树的一种方法是使用前序遍历。当我们遇到一个非空节点时，我们可以记录下这个节点的值。如果它是一个空节点，我们可以使用一个标记值记录，例如 #。

     _9_
    /   \
   3     2
  / \   / \
 4   1  #  6
/ \ / \   / \
# # # #   # #

例如，上面的二叉树可以被序列化为字符串 "9,3,4,#,#,1,#,#,2,#,6,#,#"，其中 # 代表一个空节点。

给定一串以逗号分隔的序列，验证它是否是正确的二叉树的前序序列化。编写一个在不重构树的条件下的可行算法。

每个以逗号分隔的字符或为一个整数或为一个表示 null 指针的 '#'。

你可以认为输入格式总是有效的，例如它永远不会包含两个连续的逗号，比如 "1,,3" 。

示例 1:

输入: "9,3,4,#,#,1,#,#,2,#,6,#,#"
输出: true

示例 2:

输入: "1,#"
输出: false

示例 3:

输入: "9,#,#,1"
输出: false

---

72ms

class Solution {
    func isValidSerialization(_ preorder: String) -> Bool {
        let set = CharacterSet(charactersIn: ",")
        let preorderArr = preorder.components(separatedBy: set)
        
        if preorderArr.count % 2 == 0 {
            return false
        }
        
        var i = 1
        
        for c in preorderArr {
            if i == 0 {
                return false
            }
            if c == "#" {
                 i -= 1
            }else {
                i += 1
            }
        }
        
        return i == 0
    }
}

---

76ms

class Solution {
    func isValidSerialization(_ preorder: String) -> Bool {
        let arr = preorder.components(separatedBy:",")
        if arr[0] == "#" && arr.count != 1 {
            return false
        }
        var count = 0
        
        for i in 0..<arr.count {
            let str = arr[i]
            
            if str == "#" {
                count += 1
            } else {
                count -= 1
            }
            
            if count == 1 && i != arr.count - 1 {
                return false
            }
        }
        
        return count == 1
    }
}

---

80ms

class Solution {
    func isValidSerialization(_ preorder: String) -> Bool {
 let nodes = preorder.components(separatedBy: ",")
        var stack = [String]()
        
        for i in 0..<nodes.count {
            let node = nodes[i]
            while node == "#" && !stack.isEmpty && stack.last! == "#" {
                stack.removeLast()
                if stack.isEmpty {
                    return false
                }
                stack.removeLast()
            }
            stack.append(node)
        }
        
        return stack.count == 1 && stack.last! == "#"
    }
}

---

140ms

class Solution {
    func isValidSerialization(_ preorder: String) -> Bool {
        let res = preorder.components(separatedBy: ",")
        var diff = 1
        
        for r in res {
            diff -= 1
            if diff < 0 {
                return false
            }
            if r != "#" {
                diff += 2
            }
        }
        
        return diff == 0
    }
}