leetcode 331. Verify Preorder Serialization of a Binary Tree

逃离我推掉我的手 2022-08-20 15:14 112阅读 0赞

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`

class Solution {
    public:
    	bool isValidSerialization(string preorder) {
    		if(preorder.compare("#")==0)
    			return true;
    		int comma=preorder.find_first_of(',');
    		vector<string>quene;
    		while(comma!=string::npos)
    		{
    			string str=string(preorder.begin(),preorder.begin()+comma);
    			preorder.erase(preorder.begin(),preorder.begin()+comma+1);
    			comma=preorder.find_first_of(',');
    			//string str2="#";
    			if(str.compare("#")==0)
    			{
    				if(quene.size()==1&&preorder.compare("#")==0)
    					return true;
    				else
    				{
    					if(quene.empty())
    						return false;
    					quene.pop_back();
    				}
    			}
    			else
    				quene.push_back(str);
    		}
    		return false;
    	}
    };

accepted