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

这个题想到用栈来做就问题不大了，然后就是想清楚程序结束的条件。

public class Solution {    public boolean isValidSerialization(String preorder) {        Stack<String> stack=new Stack<String>();        String[] strs =preorder.split(",");        int i=0;        while(i<strs.length){            stack.push(strs[i]);            while(stack.peek().equals("#")){                stack.pop();                if(stack.isEmpty()){                    return i==strs.length-1;                }                String pop=stack.pop();                if(pop.equals("#")){                    stack.pop();                    stack.push("#");                }else{                    stack.push(pop);                    stack.push("#");                    break;                }            }                             i++;        }        return false;    }}