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:

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

Example 2:

Input: "1,#"
Output: false

Example 3:

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

Solution

class Solution {
    public boolean isValidSerialization(String preorder) {
        String[] nodes = preorder.split(",");
        if(nodes.length == 1) {
            return Objects.equals(nodes[0], "#");
        } else if(Objects.equals(nodes[0], "#")) {
            return false;
        }
        Stack<String> stack = new Stack<>();
        stack.push(nodes[0]);
        stack.push(nodes[0]);
        for(int i = 1; i < nodes.length; i++) {
            if(stack.empty()) return false;
            stack.pop();
            if(!Objects.equals(nodes[i], "#")) {
                stack.push(nodes[i]);
                stack.push(nodes[i]);
            }
        }
        return stack.empty();
    }
}