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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: $O(1)$