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Assume a, e, i, o, u variables mean the number of elements which starts with a, e, i, o, u. Take below as example, length is 5 now. There are a number of elements which starts with a, e number of elements which starts with e, etc.
When extend the length to 6.
When adding a, a can be added in any elements.
When adding e, e can be added in front of elements starting with e, i, o, u.
When adding i, i can be added in front of elements starting with i, o, u.
When adding o, o can be added in front of elements starting with o, u.
When adding u, u can be added in front of elements starting with u.
The elements are lexical order. This is an important criteria.