155-最小栈

设计一个支持 push，pop，top 操作，并能在常数时间内检索到最小元素的栈。

push(x) -- 将元素 x 推入栈中。
pop() -- 删除栈顶的元素。
top() -- 获取栈顶元素。
getMin() -- 检索栈中的最小元素。
示例:

MinStack minStack = new MinStack();
minStack.push(-2);
minStack.push(0);
minStack.push(-3);
minStack.getMin();   --> 返回 -3.
minStack.pop();
minStack.top();      --> 返回 0.
minStack.getMin();   --> 返回 -2.

解法1：
class MinStack {

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

    public void push(int x) {
        if (stack.isEmpty()&&minStack.isEmpty()){
            stack.push(x);
            minStack.push(x);
        }else {
            int y=minStack.peek();
            stack.push(x);
            if (y<x){
                minStack.push(y);
            }else {
                minStack.push(x);
            }
        }
    }

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

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

    public int getMin() {
        return minStack.peek();
    }
}


解法2：class MinStack {

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

    public void push(int x) {
        if (stack.isEmpty()){
            stack.push(x);
            stack.push(x);
        }else {
            int y=stack.peek();
            stack.push(x);
            if (y<x){
                stack.push(y);
            }else {
                stack.push(x);
            }
        }
    }

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

    public int top() {
        return stack.get(stack.size()-2);
    }

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