foo.cpp: In function 'int main()':
foo.cpp:29:25: error: 'sort' was not declared in this scope
     sort(a1+1,a1+1+n,cmp);//di ji xiao shi ji hao wei 
                         ^
foo.cpp:27:8: warning: unused variable 'j' [-Wunused-variable]
  int i,j,k,l;
        ^
foo.cpp:27:10: warning: unused variable 'k' [-Wunused-variable]
  int i,j,k,l;
          ^
foo.cpp:27:12: warning: unused variable 'l' [-Wunused-variable]
  int i,j,k,l;
            ^
foo.cpp:41:10: warning: unused variable 'r' [-Wunused-variable]
  node* q,r;
          ^