## Verify Preorder Serialization of a Binary Tree

07/18/2016 Tree Stack

## 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:

"9,3,4,#,#,1,#,#,2,#,6,#,#"

Return true

Example 2:

"1,#"

Return false

Example 3:

"9,#,#,1"

Return false

## Solution

Result: Accepted Time: 0 ms

Here should be some explanations.

int ptr;
bool valid(char * pre)
{
if(pre[ptr] == ',') ptr++;
if(pre[ptr] == '\0')
return false;
if(pre[ptr++] =='#')
return true;
while(isdigit(pre[ptr])) ptr++;
return valid(pre) && valid(pre);
}

bool isValidSerialization(char* preorder) {
ptr = 0;
return valid(preorder) && preorder[ptr] == '\0';
}


Complexity Analytics

• Time Complexity: $O(n)$
• Space Complexity: $O(1)$