LeetCode 331. 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'#' representingnull pointer.

Note: You may assume that the input format is always valid, for example if could never contain two consecutive commas such as "1,,3" For example: "9, 3, 4, #, #, 1, #, #, 2, #, 6, #, #", return true "1, #" return false "9, #, #, 1" return false*/

Main Idea: Since the NULL will be output as "#", when we processed valid substr, we use "#" to stand for it. For example:

     _9_
    /   \
   3     2
  / \   / \
 4   1  #  6
/ \ / \   / \
# # # #   # #
     _9_
    /   \
   3     2
  / \   / \
 #   1  #  6
    / \   / \
    # #   # #     we substitute the "4, #, #" to "#"

By doing this recursively, the final result will be a only a "#" left.

#include <iostream>
#include <string>
#include <vector>
#include <stack>
using namespace std;

bool isValidSerialization(string preorder) {
  if(preorder.size() == 0) return true;
  if(preorder.size() % 2 == 0) return false;
  stack<char> nodes;
  for(int i = 0; i < preorder.size(); ++i) {
    if(preorder[i] == ',') continue;
    if(nodes.size() >= 3) {
      char tmp_1 = nodes.top();
      if(tmp_1 == '#') {
        nodes.pop();
        char tmp_2 = nodes.top();
        if(tmp_2 == '#') {
          nodes.pop();
          nodes.pop();
          nodes.push('#');
        } else {nodes.push(tmp_1);}
      }
    }
    nodes.push(preorder[i]);
  }
  return !nodes.empty() && (nodes.top() == '#');
}

int main(void) {
  string preorder_test = "9, 3, 4, #, #, 1, #, #, 2, #, 6, #, #";
  cout << isValidSerialization(preorder_test) << endl;

  string preorder_test1 = "1, #";
  cout << isValidSerialization(preorder_test1) << endl;

  string preorder_test2 = "9, #, #, 1";
  cout << isValidSerialization(preorder_test2) << endl;
}