13860번 - Letters 다국어

We build the infinite sequence of uppercase letters (A-Z) which starts with single-letter strings (A, B, …, Z), continues with two-letter strings (AA, AB, …, AZ, BA, BB, …, BZ, …, ZZ), then threeletter strings and so on. The same-length strings are ordered lexicographically. We are interested in finding which letter sits at a given index in the sequence.

The letter index, ranging between 0 and 2*10⁹ .

The letter at the specified index.