Min Stack

题目：

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.

分析：

逻辑本身不复杂，只要注意更新minVal的方法就可以

解法：

public class MinStack {
    public Stack<Integer> stackRegular, stackMin;
    public int minVal;

    public MinStack() {
        // do initialize if necessary
        stackRegular = new Stack<Integer>();
        stackMin = new Stack<Integer>();
        minVal = Integer.MAX_VALUE;
    }

    public void push(int number) {
        // write your code here
        stackRegular.push(number);
        if (number < minVal) {
            minVal = number;
        }
        stackMin.push(minVal);
    }

    public int pop() {
        // write your code here
        int res = -1;
        if (!stackRegular.empty() ) {
            res = stackRegular.peek();
            stackRegular.pop();
            stackMin.pop();

            minVal = stackMin.empty() ? Integer.MAX_VALUE : stackMin.peek();
        } 
        return res;
    }

    public int min() {
        // write your code here
        int res = -1;
        if (!stackMin.empty() ) {
            res = stackMin.peek();
        }
        return res;
    }
}