leetcode--Verify Preorder Serialization of a Binary Tree
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.
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public class Solution { public boolean isValidSerialization(String preorder) { Stack<String> stack=new Stack<String>(); String []num=preorder.split(","); int i=0; if(num[0].equals("#")&&num.length>1)return false; for(;i<num.length;i++){ if(!num[i].equals("#")){ stack.push(num[i]); }else if(stack.size()>0&&num[i].equals("#")){ stack.pop(); } else if(stack.size()==0&&num[num.length-1].equals("#")&&i==num.length-1){ return true; }else{ return false; } } return false; } }