最小栈

描述：

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.

代码实现：

class MinStack {

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

}

/**
 * Your MinStack object will be instantiated and called as such:
 * MinStack obj = new MinStack();
 * obj.push(x);
 * obj.pop();
 * int param_3 = obj.top();
 * int param_4 = obj.getMin();
 */