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This article mainly introduces the relevant knowledge of "how to describe palindromes in c language". The editor shows you the operation process through practical cases, the operation method is simple, fast and practical. I hope this article "how to describe palindromes in c language" can help you solve the problem.

Topic description

Note: (none of these palindromes have a leading 0)

The number of palindromes in 1 bit is 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, altogether

The number of palindromes in 2 digits is 11, 22, 33, 44, 55, 66, 77, 88, 99.

* excuse me: how many palindromes are there in n bits? Please write a recursive function to solve this problem!

[input form] one positive integer per line, how many bits does it represent?

[output form] one positive integer per line, representing the number of palindromes.

[sample input] 2

[sample output] 9

Enter:

three

Output:

ninety

Enter:

five

Output:

nine hundred

* * enter:

ten

Output:

900000 yuan *

Enter:

eight

Output:

9000

Enter:

one

Output:

ten

Train of thought analysis

Read this number through the for loop, reverse the data through / and% operations, and then compare whether the reversed number is equal to the original number.

Read this number through the for loop, take one number at the top and one at the end, compare whether the two numbers are equal in turn, and then remove the two numbers until there is one number (odd digits) or two digits (even digits).

Through the mathematical relationship, we can directly judge the number of digits and calculate the number of palindromes in this number.

For example: 99899

It can be divided into two halves, taking the first half of 998, if it is the number of palindromes, the second half must correspond to its corresponding position, 998 is a three-digit number. * except for the first bit (excluding leading 0), so the number corresponding to the second half has 9 choices (1-9). Other bits have 10 choices (0-9) with the corresponding position, such as the second bit and the penultimate digit (0-9).

So we can sum up the same number of digits, the number of digits is odd and odd, the number of palindromes is 9 * 10 ^ (nhand 2), note that nprime 2 is an integer, and the number of digits is even is 910 ^ (nlap 2-1), so the number of palindromes of 5 digits is 910, 10, 900.

Note that the number of digits is 1, there are 10 (0-9), which requires special treatment.

Code description

1. The first way of thinking:

# include # include int reverse (long int iGrainLong int * terminate) / / Recursive function reverse {if (I)

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