Reads the same backwards

Say whether a whole number reads the same forwards and backwards. 1221 does. 123 does not.

Worked example#

The simplest correct method uses the reversal you already know: reverse the number and compare it with the original.

For 1221:

1221 equals 1221, so it is a palindrome.

For 123 the reversal is 321, which is not 123.

Keep a copy of the original before the loop. The loop destroys n on its way to zero, so comparing against n afterwards compares 1221 with 0 and every number fails.

A faster way, and why it is fiddly#

You can reverse only the second half and compare it with the first, stopping when the reversed part is at least as large as what remains. It does half the work and it has an awkward case: an odd number of digits leaves a middle digit that belongs to neither half and must be discarded before comparing.

Worth knowing. Not worth starting with.

Negative numbers#

A negative number is never a palindrome here, because -121 reversed reads 121-, which is not a number. The constraints keep the input at zero or above, so there is nothing to handle, but it is the first thing an interviewer asks.

Constraints#

• 0 <= n <= 2000000000