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    1. 初始化：创建stack和minStack两个Deque，并将Integer.MAX_VALUE压入minStack;
    2. 入栈：将value压入stack,将Math.min(minStack.peek(),x)压入minStack;
    3. 出栈：stack和minStack同时出栈；
    4. 找最小值：minStack.peek();
    5. 顶端元素：stack.peek();