Stack: List से PUSH, POP

stack = []
stack.append(10)     # push
stack.append(20)
stack.append(30)
print('Stack:', stack, '| top =', stack[-1])
print('Popped:', stack.pop())   # removes the LAST item added
print('Stack now:', stack)

def isEmpty(stk):
    return len(stk) == 0

def push(stk, item):
    stk.append(item)

def pop(stk):
    if isEmpty(stk):
        return 'Underflow'          # nothing to remove
    return stk.pop()

def peek(stk):
    if isEmpty(stk):
        return 'Underflow'
    return stk[-1]

def display(stk):
    if isEmpty(stk):
        print('Stack is empty')
    else:
        for item in reversed(stk):  # top first
            print(item)

s = []
push(s, 'A'); push(s, 'B'); push(s, 'C')
display(s)
print('Peek:', peek(s))
print('Pop:', pop(s), pop(s), pop(s))
print('Pop on empty:', pop(s))

MAX = 3
def push(stk, item):
    if len(stk) == MAX:
        print('Overflow! cannot push', item)
    else:
        stk.append(item)
s = []
for x in [5, 6, 7, 8]:
    push(s, x)
print(s)

def reverse_string(text):
    stk = []
    for ch in text:
        stk.append(ch)        # push every character
    out = ''
    while stk:
        out += stk.pop()      # pop gives them back in reverse
    return out
print(reverse_string('PYTHON'))

def balanced(expr):
    pairs = {')': '(', ']': '[', '}': '{'}
    stk = []
    for ch in expr:
        if ch in '([{':
            stk.append(ch)
        elif ch in ')]}':
            if not stk or stk.pop() != pairs[ch]:
                return False
    return len(stk) == 0
for e in ['(a+b)*[c-d]', '((a+b)', '{[()]}', '(]']:
    print(e, '->', balanced(e))

# Board pattern: push numbers divisible by 5 from a list; then pop & display
N = [12, 15, 30, 7, 45, 9]
stack = []
def PUSH_FIVE(lst):
    for n in lst:
        if n % 5 == 0:
            stack.append(n)
def POP_ALL():
    while stack:
        print(stack.pop(), end=' ')
    print()
    print('Stack Empty')
PUSH_FIVE(N)
print(stack)
POP_ALL()

# Board pattern: dictionary of students -> push names with marks > 75
R = {'Om': 76, 'Jai': 45, 'Bob': 89, 'Ali': 65, 'Anu': 90, 'Tom': 82}
ST = []
def PUSH(S, R):
    for name in R:
        if R[name] > 75:
            S.append(name)
def POP(S):
    if S:
        return S.pop()
    return None
PUSH(ST, R)
while True:
    x = POP(ST)
    if x is None:
        break
    print(x, end=' ')
print()

def menu():
    stk = []
    while True:
        print('1.Push 2.Pop 3.Peek 4.Display 5.Exit')
        ch = int(input('Choice: '))
        if ch == 1:
            stk.append(input('Item: '))
        elif ch == 2:
            print('Popped:', stk.pop() if stk else 'Underflow')
        elif ch == 3:
            print('Top:', stk[-1] if stk else 'Underflow')
        elif ch == 4:
            print(stk[::-1])
        else:
            break
menu()