给定一个数字字符串，返回所有这些数字可以表示的字母组合。
Given a digit string, return all possible letter combinations that the number could represent.
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.
将这些字母组合在一起就会拼出“妈妈”，一个对我来说意味着整个世界的词。
Put them all together, they spell "MOTHER, " A word that means the world to me.
字母组合字符串(composition string)是指字母组合窗口中的当前文本。
The composition string is the current text in the composition window.
字母组合字符串(composition string)是指字母组合窗口中的当前文本。
The composition string is the current text in the composition window.
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