View Compilation Error

0_0_37813335_10824.cpp: In function 'void DFS(int)':
0_0_37813335_10824.cpp:18:10: error: invalid operands of types 'll {aka long long int}' and 'void' to binary 'operator+'
   sum[u] += DFS( v ) ;  
          ^
0_0_37813335_10824.cpp:18:10: error:   in evaluation of 'operator+=(ll {aka long long int}, void)'