LeetCode Verify Preorder Serialization of a Binary Tree

Description:

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".

Solution:

Use a stack. Every time we get a "a # #", replace it with "#", and continuing this in the stack.

Be careful of the edge case.

import java.util.Stack;

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

		return true;
	}
}