[LeetCode] Verify Preorder Serialization of a Binary Tree

Posted 2020-11-10 immjc

 

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 nullpointer.

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

根据二叉树性质，空结点比实结点大一。

class Solution {
public:
    bool isValidSerialization(string preorder) {
        stringstream ss(preorder);
        string val;
        int diff = 1;
        while (getline(ss, val, ‘,‘))
        {
            if (diff == 0)
                return false;
            if (val == "#")
                diff--;
            else
                diff++;
        }
        return diff == 0;
    }
};