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.

Example:

MinStack minStack = new MinStack();
minStack.push(-2);
minStack.push(0);
minStack.push(-3);
minStack.getMin();   --> Returns -3.
minStack.pop();
minStack.top();      --> Returns 0.
minStack.getMin();   --> Returns -2.

Solution:

public class MinStack {
    private ArrayList<Integer> l ;
    private ArrayList<Integer> min;
    /** initialize your data structure here. */
    public MinStack() {
        l  = new ArrayList<Integer>();
        min = new ArrayList<Integer>();
    }

    public void push(int x) {
        l.add(x);
        if(min.size() == 0) min.add(x);
        else if(min.get(min.size()-1) >= x) min.add(x);

    }

    public void pop() {
        int r = l.remove(l.size()-1);
        if(min.get(min.size()-1) == r) min.remove(min.size()-1);
    }

    public int top() {
        return l.get(l.size()-1);
    }

    public int getMin() {
        return min.get(min.size()-1);
    }
}

/**
 * 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();
 */