# Question

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


# Solution

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