[LeetCode] Verify Preorder Serialization of a Binary Tree

发布时间：2020-12-14 04:48:41 所属栏目：大数据来源：网络整理

导读：? 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/ / / # # # #

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: 
Output: "9,#"true

Example 2:

Input: 
Output: 
"1,#"false

Example 3:

Input: 
Output: "9,1"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;
    }
};

（编辑：李大同）

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