README.md

stacks

• stacks are last in, first out (LIFO) abstract structures, where the newest element is first one to be removed from the structure.

• a stack support push and pop at O(1), and they be implemented with arrays or singly linked list.

• stacks are useful in depth-first search algorithms, where you can push temp data as you recurse, and remove them from the (memory or data structure) stack as you backtrace.

• to keep find_min() at O(1), you can keep track of "prior minimum" when pushing and poping:

class Stack:

    def __init__(self):
        self.stack = []
        self.min = None

    def push(self, val: int) -> None:
        self.stack.append((val, self.min))
        if self.min is not None:
            self.min = min(self.min, val)
        else:
            self.min = val

    def pop(self) -> None:
        if self.stack:
            (val, prior_min) = self.stack.pop()
            self.min = prior_min
            return val
        
    def top(self) -> int:
        if self.stack:
            return self.stack[-1][0]

    def get_min(self) -> int:
        return self.min