【Leetcode】Verify Preorder Serialization of a Binary Tree

题目链接:https://leetcode.com/problems/verify-preorder-serialization-of-a-binary-tree/

题目:

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:
"9,3,4,#,#,1,#,#,2,#,6,#,#"
Return true

Example 2:
"1,#"
Return false

Example 3:
"9,#,#,1"
Return false

思路:

两个#可以退一个栈,最后如果栈只剩一个#则true否则false

算法

public boolean isValidSerialization(String preorder) {  
    Stack<String> stack = new Stack<String>();  
    String ps[] = preorder.split(",");  
    for (int i = 0; i < ps.length; i++) {  
        stack.push(ps[i]);  
        while (!stack.isEmpty() && stack.peek().equals("#") && stack.size() >= 3  
                && stack.get(stack.size() - 2).equals("#") && !stack.get(stack.size() - 3).equals("#")) {  
            stack.pop();  
            stack.pop();  
            stack.pop();  
            stack.push("#");  
        }  
    }  
    return stack.size() == 1 && stack.peek().equals("#");  
}  
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