Problem
题目大意:验证二叉树前序遍历的序列化是否合法。
https://leetcode.com/problems/verify-preorder-serialization-of-a-binary-tree/description/
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
Credits:
Special thanks to @dietpepsi for adding this problem and creating all test cases.
Solution: Recursion
- If a node is not null, it must has two children, thus verify left subtree and right subtree recursively.
- If a not is null, the current char must be ‘#’
Time complexity: O(n)
Space complexity: O(h)
C++
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 |
// Author: Huahua // Running time: 5 ms class Solution { public: bool isValidSerialization(string preorder) { int pos = 0; return isValid(preorder, pos) && pos == preorder.length(); } private: bool isValid(const string& s, int& pos) { if (pos >= s.length()) return false; if (isdigit(s[pos])) { while (isdigit(s[pos])) ++pos; return isValid(s, ++pos) && isValid(s, ++pos); } return s[pos++] == '#'; } }; |