View Compilation Error

0_0_38588143_260.cpp: In function 'void work()':
0_0_38588143_260.cpp:17:21: error: 'a' was not declared in this scope
         len[i - pre[a[i]]] ++;
                     ^
0_0_38588143_260.cpp:25:18: error: 'a' was not declared in this scope
         s.insert(a[n - i + 2]);
                  ^