Min Stack(lintcode 12)

Description

Implement a stack with min() function, which will return the smallest number in the stack.

It should support push, pop and min operation all in O(1) cost.

Notice:
min operation will never be called if there is no number in the stack.

Example

push(1)
pop()   // return 1
push(2)
push(3)
min()   // return 2
push(1)
min()   // return 1

Interface

public class MinStack {

    public MinStack() {
        // do initialize if necessary
    }

    public void push(int number) {
        // write your code here
    }

    public int pop() {
        // write your code here
    }

    public int min() {
        // write your code here
    }
}

Solution

public class MinStack {
    private Stack<Integer> stack;
    private Stack<Integer> minStack;

    public MinStack() {
        // do initialize if necessary
        stack = new Stack<Integer>();
        minStack = new Stack<Integer>();
    }

    public void push(int number) {
        // write your code here
        stack.push(number);
        if (minStack.isEmpty()) {
            minStack.push(number);
        } else {
            minStack.push(Math.min(number, minStack.peek()));
        }
    }

    public int pop() {
        // write your code here
        minStack.pop();
        return stack.pop();
    }

    public int min() {
        // write your code here
        return minStack.peek();
    }
}