123454321
12345654321
123546321
12344321

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We can make a multiplication from the outset, but we can also use mathematical induction which, in this case, is quicker. The first line, the amount of $1$ in square is two digits, hence the square being from 1 to 2 then reverse (without repetition of 2) to 1. Similarly, in the second line, the amount of $1$ in square is three digits, hence the square being from 1 to 3 then reverse (without repetition of 3) to 1.

Hence, the amount of $1$ in square is five digits, hence the square being from 1 to 5 then reverse (without repetition of 5) to 1, or by spelling out, $123454321$