MinStack.java

package stack;

import java.util.Stack;

/**
 * Created by gouthamvidyapradhan on 08/03/2017.
 * Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.
 * <p>
 * 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.
 */
public class MinStack {
    class Node {
        int value, min;

        Node(int value, int min) {
            this.value = value;
            this.min = min;
        }
    }

    private Stack<Node> stack = new Stack<>();

    /**
     * Main method
     *
     * @param args
     * @throws Exception
     */
    public static void main(String[] args) throws Exception {
        MinStack minStack = new MinStack();
        minStack.push(-2);
        minStack.push(0);
        minStack.push(-3);
        System.out.println(minStack.getMin());
        minStack.pop();
        System.out.println(minStack.top());
        System.out.println(minStack.getMin());
    }

    public MinStack() {

    }

    public void push(int x) {
        Node node;
        if (!stack.isEmpty()) {
            Node top = stack.peek();
            node = new Node(x, Math.min(top.min, x));
        } else {
            node = new Node(x, x);
        }
        stack.push(node);
    }

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

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

    public int getMin() {
        return stack.peek().min;
    }

}