331. Verify Preorder Serialization of a Binary Tree

Problem：

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.

Solution：

class Solution {
public:
bool isValidSerialization(string preorder) {
stack<char> a;
for (int i = 0; i < preorder.size(); i++)
{
if (preorder[i] == ',')
{
continue;
}
if (preorder[i] != '#')
{
a.push(preorder[i]);
}
else
{
while (!a.empty())
{
if (a.top() != '#')
{
break;
}
a.pop();
if (a.top() == '#' || a.empty())
{
return false;
}
else
{
a.pop();
}


}
a.push('#');
}
}
if (a.top() == '#' && a.size() == 1)
{
return true;
}
else
{
return false;
}
}
};