## 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:

```Input: `"9,3,4,#,#,1,#,#,2,#,6,#,#"`
Output: `true````

Example 2:

```Input: `"1,#"`
Output: `false`
```

Example 3:

```Input: `"9,#,#,1"`
Output: `false````

## Rust Solution

``````struct Solution;

impl Solution {
fn is_valid_serialization(preorder: String) -> bool {
let mut stack: Vec<char> = vec![];
for tok in preorder.split(',') {
if tok == "#" {
while let Some('#') = stack.last() {
stack.pop();
if stack.pop().is_none() {
return false;
};
}
stack.push('#');
} else {
stack.push('\$');
}
}
stack == vec!['#']
}
}

#[test]
fn test() {
let preorder = "9,3,4,#,#,1,#,#,2,#,6,#,#".to_string();
let res = true;
assert_eq!(Solution::is_valid_serialization(preorder), res);
let preorder = "1,#".to_string();
let res = false;
assert_eq!(Solution::is_valid_serialization(preorder), res);
let preorder = "9,#,#,1".to_string();
let res = false;
assert_eq!(Solution::is_valid_serialization(preorder), res);
let preorder = "1,#,#,#,#".to_string();
let res = false;
assert_eq!(Solution::is_valid_serialization(preorder), res);
}
``````

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