What is a stack, in simple terms: last in, first out.

Algorithm principle

• Push to the stack (PUSH) algorithm

1. If TOP≥n, an overflow message will be given and an error handling will be made (before entering the stack, first check whether the stack is full, overflow if it is full; do 2 if it is not full)
2. Set TOP=TOP+1 (add 1 to the stack pointer to point to the stack address)
3. S(TOP)=X, end (X is the new element on the stack)

• Unstack (POP) algorithm

1. If TOP≤0, an underflow information will be given, and an error handling will be made (check whether the stack is empty before exiting the stack, if it is empty, it will underflow; if it is not empty, then it will be 2)
2. X=S(TOP), (the elements after unstacking are assigned to X)
3. TOP=TOP-1, end (stack pointer minus 1, pointing to the top of the stack)

Algorithm implementation

# - * - coding : utf -8  - * -

__author__ =  'Bitter leaves'


class  Stack : 
    def __init__ ( self , size = 30 ) :
        # Initialize stack size
        self . size = size        

        # Initialize the stack list
        self . stack =  [ ]        
        
        # Initialize the stack default top value
        Self . Top =  - . 1

    # Set stack size
    def set_size ( self , size ) : 
        self . size = size    

    # Determine whether the stack is empty
    def is_empty ( self ) : 
        res = False
         if self . top ==  - 1 : 
            res = True

        return res    
        
    # Determine if the stack is full
    def is_full ( self ) : 
        res = False
         if self . top +  1  == self . size : 
            res = True

        return res    
    
    # Print everything in the stack
    def show ( self ) : 
        print ( self . stack )     # into the stack
    def push ( self , obj ) : 
        if self . is_full ( ) :             
            raise Exception ( "The stack is full..." )         
        else : 
            self . stack . append ( obj ) 
            self . top +=  1

    # Unstack
    def pop ( self ) : 
        if self . is_empty ( ) :      
            raise Exception ( "The stack is empty..." ) 
        else : 
            self . top -=  1 
            return self . stack . pop ( )
        
        
if __name__ ==  "__main__" :

    print ( "stack implementation example" )     
    # Initially a stack instance with a length of 5
    stack =  Stack ( 5 )    
    
    # Stack integer 1 - 5 
    for index in  the Range ( 1 ,  6 ) : 
        Stack . The Push ( index )    

    # Print the contents of the next stack
    Stack . Show ( )    
    
    # The stack , the value of the data should be . 5 
    data = Stack . POP ( ) 
    Print ( data )    
    
    # Print the contents of the stack, which should be [ 1 , 2 , 3 , 4 ] 
    stack . Show ( )