LeetCode刷题（C++）——Verify Preorder Serialization of a Binary Tree（Medium）

旧城等待， 2022-06-15 01:29 166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) {
            int position = 1;
            for(int i=0; i<preorder.size(); ++i){
                if(preorder[i]== ','){
                    preorder[i] = ' ';
                }
            }
            istringstream out(preorder);
            string str;
            while(out>>str){
                if(--position < 0){
                    return false;
                }
                if(str != "#"){
                    position += 2;
                }
            }
            return position == 0;
        }
    };