The substring() method in Java is a powerful tool for extracting portions of a string. It allows you to create a new string based on the characters within a specified range of an existing string.
第二个方法，是Code Granker上面的，利用了高中学排列组合时候的概念。 有个‘abc’，那么总共可以看成'a_b_c' 5个字符位置 ...
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