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  1. #include <stdio.h>
  2. #include <string.h>
  3. #include <ctype.h>
  4.  
  5. // step 1. reverse the target sequence
  6. // "abc 123 xyz" ==> "cba 123 xyz"
  7. //  ^^^               ^^^
  8.  
  9. // step 2. Reverse the entire sequence
  10. // "cba 123 xyz" ==> "zyx 321 abc"
  11.  
  12. // step 3 Reverse the non-target sequence, not including the trailing space
  13. // "zyx 321 abc " => "123 xyz abc"
  14. //  ^^^^^^^           ^^^^^^^
  15. //
  16. // repeat this (recursion or iteration, makes no difference) for
  17. //  the non-target segment, adjusting the end-marker to do so.
  18.  
  19. void reverse_letters(char *beg, char *end)
  20. {
  21.     while (beg < --end)
  22.     {
  23.         char tmp = *beg;
  24.         *beg++ = *end;
  25.         *end = tmp;
  26.     }
  27. }
  28.  
  29. char *reverse_words(char *beg, char *end)
  30. {
  31.     // find whitespace. if none then we're done.
  32.     char *p = beg;
  33.     for (; p != end && !isspace((unsigned char)*p); ++p);
  34.  
  35.     if (p != end)
  36.     {
  37.         reverse_letters(beg, p);
  38.         reverse_letters(beg, end);
  39.         reverse_letters(beg, beg + (end - p - 1));
  40.         reverse_words(beg, beg + (end - p - 1));
  41.     }
  42.  
  43.     return beg;
  44. }
  45.  
  46. int main()
  47. {
  48.     char words[] = "abc 123 xyz";
  49.     reverse_words(words, words + strlen(words));
  50.     puts(words);
  51.     return 0;
  52. }
  53.  
Success #stdin #stdout 0s 4512KB
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stdin
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Standard input is empty
stdout
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xyz 123 abc
https://ideone.com/wtndWM
language:
C (gcc 8.3)
created:
6 years ago
visibility:
public

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