Returns:;; An iterator referring to the first element of the container, or if the container is empty the past-the-end value for the container.
Returns:;; An iterator referring to the first element of the container, or if the container is empty the past-the-end value for the container.返回：指向容器第一个元素的迭代器，如果容器为空则返回容器的尾后迭代器。
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Returns:;; An iterator referring to the first element of the container, or if the container is empty the past-the-end value for the container.返回：指向容器第一个元素的迭代器，如果容器为空则返回容器的尾后迭代器。