331. Verify Preorder Serialization of a Binary Tree

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

Credits:

Special thanks to @dietpepsi for adding this problem and creating all test cases.

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public class Solution {
private String [] str;
private int i;
private boolean test(){
if(i>=str.length)return false;
if(str[i].equals("#"))return true;
i++;
if(test()==false)return false;
i++;
if(test()==false)return false;
return true;
}
public boolean isValidSerialization(String preorder) {
i=0;
str=preorder.split(",");
boolean t = test();
if(t==false)return false;
if(i==str.length-1)return true;
return false;
}
}