bool isValidParenthesis (string expression) {
  stack<char> s;
  for (int i = 0; i < expression.length(); i++) {
    char ch = expression[i];
    // if opening bracket push it to stack
    // if closing bracket, pop from stack, check if the popped element is the                 matching opening bracket
    if (ch == '(' || ch =='{' || ch == '[') {
      s.push(ch);
    }
    else {
      // for closing bracket
      if (!s.empty()) {
        char top = s.top();
        if ((ch == ')' && top == '(') || (ch == '}' && top == '{') || (ch == ']' &&               top == '[')) {
          s.pop();
        }
      }
      else {
        return false;
      }
    }
  }
  if (s.empty()) {
    return true;
  }
  else {
    return false;
  }
}

# //Used in call stack, undo/redo, routing 
 
# //Array implenentation (just use pop/push or shift/unshift) 
# //Recall that pop/push is more efficient than shift/unshift 
# //Indexes take up memory so linked list implenetation will be better 
 
# //Single linked implenetation of stack:

class Node: 
  def __init__(self, value, next=None): 
    self.value = value 
    self.next = next 

class Stack: 
  def __init__(self, first=None, last=None, size=0): 
    self.first = first
    self.last = last 
    self.size = size 
  
  #Similar to shift or insert(0, value). Adds to stack and returns size. Recall stack is LIFO  
  def push(self, value): 
    new_node = Node(value) 
    if self.first == None: 
      self.first = new_node 
      self.last = new_node 
    else: 
      #stores current 1st property on stack 
      temp = self.first 
      #reset 1st property to be newly created one 
      self.first = new_node 
      #set new next property 
      self.first.next = temp 
    #increment stack 
    self.size += 1  
    return self.size 
  
  #in python, this would be pop(0). Similar to unshift. Removes first element on stack  
  def pop(self): 
    #special case if stakc is empty 
    if self.first == None: 
      return None 
      
    temp = self.first 
    #if there is only 1 node, set 1st node and last property to be None 
    if self.first == self.last: 
      self.last = None
      #self.first = None will be set in next line 
    
    #for all scenarios except empty stack 
    self.first = self.first.next 
    self.size -= 1 
    return temp.value 
    
    #useful to visualize reverse() or any other function works 
  def print_stack_to_list(self): 
    result = []
    current = self.first 
    while current: 
      result.append(current.value)
      current = current.next 
    print(result)


s = Stack() 
s.push(1) 
s.push(2)
s.push(3)
s.print_stack_to_list() # [3, 2, 1]
s.pop()
s.print_stack_to_list() #[2, 1]



# //these methods deal with 1st node bc thats what stacks do (LIFO) 
# // Big O: 
# // Insertion and Removal: O(1) **this is mainly what stacks are good for 
# // Access and Search: O(N)