【Leetcode】Verify Preorder Serialization of a Binary Tree
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题目链接:https://leetcode.com/problems/verify-preorder-serialization-of-a-binary-tree/
题目:
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
思路:
两个#可以退一个栈,最后如果栈只剩一个#则true否则false
算法:
public boolean isValidSerialization(String preorder) { Stack<String> stack = new Stack<String>(); String ps[] = preorder.split(","); for (int i = 0; i < ps.length; i++) { stack.push(ps[i]); while (!stack.isEmpty() && stack.peek().equals("#") && stack.size() >= 3 && stack.get(stack.size() - 2).equals("#") && !stack.get(stack.size() - 3).equals("#")) { stack.pop(); stack.pop(); stack.pop(); stack.push("#"); } } return stack.size() == 1 && stack.peek().equals("#"); }
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