LeetCode: 331. Verify Preorder Serialization of a Binary Tree

Question:

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”

Solution

public class Solution {
    public boolean isValidSerialization(String preorder) {
    	if (preorder == null || preorder.length() == 0) {
    		return false;
    	}
    	String[] str = preorder.split(",");
    	Stack<String> stack = new Stack<>();
    	for (int i = 0; i < str.length; i++) {
    		String cur = str[i];
    		while (cur.equals("#") && !stack.isEmpty() && stack.peek().equals(cur)) {
    			stack.pop();
    			if (stack.isEmpty()) {
    				return false;
    			}
    			stack.pop();
    		}
    	}
    	return (stack.size() == 1 && stack.peek().equals("#");
    }
}

Note:

用一个#来替换一个数字+两个#， 最后stack里应该只剩下一个#