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

class Solution {public:    bool isValidSerialization(string preorder) {        int n = preorder.length();    int i = 0;    int m = 1;    while (i < n && m > 0)    {    while (i < n && preorder[i] != ',')    {    i++;    }    if (preorder[i-1] == '#')    {    m--;    }    else    {    m++;    }    if (i < n)    {    i++;    }    }    if (i != n || m != 0)    {    return false;    }    return true;    }};