LeetCode 155: Min Stack

Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.

• push(x) -- Push element x onto stack.
• pop() -- Removes the element on top of the stack.
• top() -- Get the top element.
• getMin() -- Retrieve the minimum element in the stack.

class MinStack {
    // Normally store elements
    Stack<Integer> stack = new Stack<>();
    
    // Store current minimum elements
    Stack<Integer> stackMin = new Stack<>();
    
    public void push(int x) {
        stack.push(x);
        
        if (stackMin.isEmpty() || x <= stackMin.peek())
            stackMin.push(x);
    }

    public void pop() {
        int x = stack.pop();
        
        if (x <= stackMin.peek())
            stackMin.pop();
    }

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

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