155. Min Stack

两个stack，一个存正常的数，一个存min值，重点在于，如果有和当前最小值一样的新的数进来，也要推进min stack

public class MinStack {
    Stack<Integer> stack;
    Stack<Integer> min;

    /** initialize your data structure here. */
    public MinStack() {
        stack = new Stack<Integer>();
        min = new Stack<Integer>();
    }
    
    public void push(int x) {
        if(min.isEmpty() || x <= min.peek()) {
            min.push(x);
        }
        stack.push(x);
    }
    
    public void pop() {
        if(!min.isEmpty()) {
            int curMin = min.peek();
            int cur = stack.pop();
            if(curMin == cur) {
                min.pop();
            }
        }
    }
    
    public int top() {
        return stack.peek();
    }
    
    public int getMin() {
        return min.peek();
    }
}