"Leetcode" Verify preorder serialization of a Binary Tree

Title Link: https://leetcode.com/problems/verify-preorder-serialization-of-a-binary-tree/

Topic:

One-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 serializatio N of a binary tree. Find an algorithm without reconstructing the tree.

each Comma Separated value in the string must being either an integer or a character  ' # '  representing null  pointer.

you may assume", "the" input format was always valid, for example it could never contain, 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"
Returnfalse

Ideas:

Two # can back a stack, and finally if the stack is only one # then true otherwise false

Algorithm:

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 ("#");  }  

"Leetcode" Verify preorder serialization of a Binary Tree