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.

Note that all the operations have to be constant time operations.

Questions to ask the interviewer :

Q: What should getMin() do on empty stack? 
A: In this case, return -1.

Q: What should pop do on empty stack? 
A: In this case, nothing. 

Q: What should top() do on empty stack?
A: In this case, return -1

NOTE : If you are using your own declared global variables, make sure to clear them out in the constructor.

Solution:

Time: N/A
Space: O(n)

class Solution {
    private Deque<Integer> stack = new ArrayDeque<>();
    private Deque<Integer> minStack = new ArrayDeque<>();
    
    public void push(int x) {
        stack.push(x);
        if (minStack.isEmpty()) {
            minStack.push(x);
        } else {
            int curMin = minStack.peekFirst();
            if (x < curMin) {
                minStack.push(x);
            } else {
                minStack.push(curMin);
            }
        }
    }

    public void pop() {
        if (stack.isEmpty()) return;
        stack.pop();
        minStack.pop();
    }

    public int top() {
        if (stack.isEmpty()) return -1;
        return stack.peekFirst();
    }

    public int getMin() {
        if (minStack.isEmpty()) return -1;
        return minStack.peekFirst();
    }
}

class MinStack {
    private Deque<Integer> stack;
    private Deque<Integer> minStack;

    /** initialize your data structure here. */
    public MinStack() {
        stack = new ArrayDeque();
        minStack = new ArrayDeque();
    }
    
    public void push(int x) {
        stack.push(x);
        if (minStack.isEmpty()) {
            minStack.push(x);
        } else if (x < minStack.peekFirst()) {
            minStack.push(x);
        } else {
            minStack.push(minStack.peekFirst());
        }
    }
    
    public void pop() {
        stack.pop();
        minStack.pop();
    }
    
    public int top() {
        return stack.peekFirst();
    }
    
    public int getMin() {
        return minStack.peekFirst();
    }
}

/**
 * Your MinStack object will be instantiated and called as such:
 * MinStack obj = new MinStack();
 * obj.push(x);
 * obj.pop();
 * int param_3 = obj.top();
 * int param_4 = obj.getMin();
 */