What is the O(n) solution?

This is O(n logn) and it can be solved in O(n)

sizeof(char) is 1 by definition.
The memory is written to, so it can't be const.
The pointer isn't updated, so it should be const.

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char * const res = malloc(len);
shouldn't be: const char* res = malloc(len*sizeof(char)); ?

If 's' is very large, it would take a lot of time, O(len(s)). Instead, try to check len(s) in the for loop. This way it should be O(1) approximately in time