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 {

    ArrayList<Integer> stack = null;
    ArrayList<Integer> minArray = null;
    int curMin = Integer.MAX_VALUE;

    /** initialize your data structure here. */
    public MinStack() {
        stack = new ArrayList<Integer>();
        minArray = new ArrayList<Integer>();
    }

    public void push(int x) {
        stack.add(x);

        curMin = Integer.min(curMin, x);
        minArray.add(curMin);
    }

    public void pop() {
        if(stack.size() > 0) {
            stack.remove(stack.size() - 1);
            minArray.remove(minArray.size() - 1);
            if(minArray.size() > 0) {
                curMin = minArray.get(minArray.size() - 1);    
            } else {
                curMin = Integer.MAX_VALUE;
            }

        }
    }

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

    public int getMin() {
        return curMin;
    }
}

/**
 * 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();
 */