Problem #155: Min Stack

Intuition & Strategy Breakdown

Maintain two stacks: a main value stack and a min stack that stores the current minimum element corresponding to each stack level.

Python Solution

class MinStack:
    def __init__(self):
        self.stack = []
        self.min_stack = []

    def push(self, val: int) -> None:
        self.stack.append(val)
        val = min(val, self.min_stack[-1] if self.min_stack else val)
        self.min_stack.append(val)

    def pop(self) -> None:
        self.stack.pop()
        self.min_stack.pop()

    def top(self) -> int:
        return self.stack[-1]

    def getMin(self) -> int:
        return self.min_stack[-1]

Java Solution

class MinStack {
    private Stack stack;
    private Stack minStack;

    public MinStack() {
        stack = new Stack<>();
        minStack = new Stack<>();
    }

    public void push(int val) {
        stack.push(val);
        val = Math.min(val, minStack.isEmpty() ? val : minStack.peek());
        minStack.push(val);
    }

    public void pop() {
        stack.pop();
        minStack.pop();
    }

    public int top() {
        return stack.peek();
    }

    public int getMin() {
        return minStack.peek();
    }
}