stack.py

class Stack:
    def __init__(self):
        self.__stack = []

    def push(self, x):
        self.__stack.append(x)

    def pop(self):
        return self.__stack.pop()

    def peek(self):
        return self.__stack[len(self.__stack) - 1]

    def is_empty(self):
        return len(self.__stack) == 0

def solve(array):
    s = Stack()
    idx = 0

    while idx < len(array):
        elem = array[idx]
        if s.is_empty():
            s.push(elem)
        else:
            while not s.is_empty() and s.peek() < elem :
                print(s.peek(), elem)
                s.pop()
            
            s.push(elem)
        idx += 1
    
    while not s.is_empty():
        print(s.peek(), -1)
        s.pop()

if __name__ == '__main__':
    solve([1, 5, 2, 7 , 1, 2])