331. Verify Preorder Serialization of a Binary Tree

One way to serialize a binary tree is to use preorder 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 ‘#’.

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 preorder, return true if it is a correct preorder traversal serialization of a binary tree.

It is guaranteed that 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”.

Note: You are not allowed to reconstruct the tree.
 

Example 1:

Input: preorder = “9,3,4,#,#,1,#,#,2,#,6,#,#”
Output: true

Example 2:

Input: preorder = “1,#”
Output: false

Example 3:

Input: preorder = “9,#,#,1”
Output: false

Constraints:

• $1<=preorder.length<=10^4$
• preorder consist of integers in the range [0, 100] and ‘#’ separated by commas ‘,’.

From: LeetCode
Link: 331. Verify Preorder Serialization of a Binary Tree

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Solution:

Ideas:

• Handling Digits: The code skips over a sequence of digits when it encounters a non-null node.
• Slot Calculation: After processing a node, the slots are updated correctly, ensuring that the tree structure is valid.

Code:

bool isValidSerialization(char* preorder) {
    int slots = 1; // initially one slot for the root
    
    for (int i = 0; preorder[i] != '\0'; ) {
        if (slots == 0) {
            return false;
        }

        if (preorder[i] == ',') {
            i++;
            continue;
        }

        if (preorder[i] == '#') {
            // Consume one slot
            slots--;
            i++;
        } else {
            // If it's a number, consume one slot but add two new slots
            while (preorder[i] != ',' && preorder[i] != '\0') {
                i++;
            }
            slots++; // Consuming one and adding two is a net increase of one slot
        }
    }
    
    // All slots should be used up if it's a valid serialization
    return slots == 0;
}