Stack implementation:

class Stack:
    def pop(self):
        return self.items.pop()

    def __init__(self):
        self.items = []

    def push(self, item):
        return self.items.append(item)

    def size(self):
        return len(self.items)

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

    def isEmpty(self):
        return len(self.items) == 0

Code for given problem

def ngr(arr):
    result = Stack()
    stack = Stack()
    for elem in arr[::-1]:
        if not stack.isEmpty() and stack.peek() < elem:
            while not stack.isEmpty() and stack.peek() <= elem:
                stack.pop()
        if not stack.isEmpty() and stack.peek() > elem:
            result.push(stack.peek())
        if stack.isEmpty():
            result.push(-1)
        stack.push(elem)
    print(arr)
    print(result.items[::-1])


ngr([0,1, 3, 2, 4])
# output
# [0, 1, 3, 2, 4]
# [1, 3, 4, 4, -1]