【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("#");
}