331. Verify Preorder Serialization of a Binary Tree

Q:

One way to serialize a binary tree is to use pre-oder 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

A:
 *******就是建立一个stack，来模拟深度，如果是visiting left child, 则stack上放1， 否则放 -1.
******************
每次看见一个 # ，表示结束了一个分支。 ********
**********直接一遍秒过，还不错，Tiger，明天犒劳自己*********

public class Solution {
    public boolean isValidSerialization(String preorder) {
        String[] A = preorder.split(",");
        Stack<Integer> stack = new Stack(); //1 for left, -1 if visiting right
        for(String cur : A){
            cur = cur.trim();
            if(! cur.equals("#")){
                stack.push(1);
            }else{
                while( ! stack.isEmpty() && stack.peek()<0){
                    stack.pop();
                }
                if( !stack.isEmpty()){
                    stack.pop();
                    stack.push(-1);
                }
            }
            
        }
        return stack.isEmpty();
    }
}