foo.c: In function 'countLetterPairs':
foo.c:57:5: warning: implicit declaration of function 'qsort' [-Wimplicit-function-declaration]
   57 |     qsort(pairs, numPairs, sizeof(PairCount), comparePairs);
      |     ^~~~~
foo.c: In function 'main':
foo.c:67:22: warning: missing terminating " character
   67 |     str[strcspn(str, "
      |                      ^
foo.c:67:22: error: missing terminating " character
foo.c:68:1: warning: missing terminating " character
   68 | ")] = '\0'; // 去掉换行符
      | ^
foo.c:68:1: error: missing terminating " character
   68 | ")] = '\0'; // 去掉换行符
      | ^~~~~~~~~~~~~~~~~~~~~~~~~
foo.c:70:26: error: expected ')' before ';' token
   70 |     countLetterPairs(str);
      |                          ^
      |                          )
foo.c:67:16: note: to match this '('
   67 |     str[strcspn(str, "
      |                ^
foo.c:70:5: error: invalid use of void expression
   70 |     countLetterPairs(str);
      |     ^~~~~~~~~~~~~~~~~~~~~
foo.c:72:14: error: expected ']' before '}' token
   72 |     return 0;
      |              ^
      |              ]
   73 | }
      | ~             
foo.c:72:14: error: expected ';' before '}' token
   72 |     return 0;
      |              ^
      |              ;
   73 | }
      | ~             

[Hydro](https://hydro.ac)提供评测服务