题目如下，输入一串数字，根据电话键盘布局输出此数字可能代表的字母组合。
Given a digit string, return all possible letter combinations that the number could represent.
我公司的国际代表会根据您的需要随时安排单独会面。
Our international representatives are always at your disposal for a personal meeting.
地质学家们根据地球的演化历史，设计出一种特殊的地质年代表。
Geologists have devised a special geological time scale in reference to the earth's evolution.
地质学家们根据地球的演化历史，设计出一种特殊的地质年代表。
Geologists have devised a special geological time scale in reference to the earth's evolution.
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