View Compilation Error

0_0_13824411_32399.cpp: In function 'int main()':
0_0_13824411_32399.cpp:49:16: error: increment of member 'Node::num' in read-only object
    else it->num++; 
                ^
0_0_13824411_32399.cpp:66:17: error: decrement of member 'Node::num' in read-only object
     else it->num--;
                 ^
0_0_13824411_32399.cpp:70:17: error: increment of member 'Node::num' in read-only object
     else it->num++;
                 ^