Question

I have always implemented the c code for reversing a string as:

1. looping I till the length of the string or till the half of its length
2. placing the pointers at the end and beginning of the string
3. swapping them one by one .

but I want an optimized code that reduces the time complexity for this problem apart from the one that I mentioned. I tried google search but did not find any relevant solution to it.

Answer 1

If by “time complexity” you’re referring to the big-O notation which excludes coefficients and lower-order terms, you will not be able to beat a simple O(n) algorithm for reversing a C string.

If you’re referring to the time it takes for a specific machine (or class of machines) to execute the operation, there is a number of approaches to optimize the reversal. Typical optimizations include loop unrolling, consuming the characters machine word by machine word instead of character by character, and a smart search for the terminating NUL character. The freely available GNU libc contains examples of such optimizations.

Some of the above optimizations, such as loop unrolling, may be automatically implemented by optimizing compilers. Others may be counter-productive on some platforms, or their speedup dependent on the size of the string. In some cases hand-written optimization can hinder the compiler’s own effort to optimize the code. The only way to be sure you’re not making things worse, develop a benchmark that covers your intended usage and meticulously benchmark your code as you progress.