View Compilation Error

0_0_13824434_3364.cpp: In function 'int main()':
0_0_13824434_3364.cpp:49:18: error: increment of member 'Node::num' in read-only object
    else (it->num)++; 
                  ^
0_0_13824434_3364.cpp:66:19: error: decrement of member 'Node::num' in read-only object
     else (it->num)--;
                   ^
0_0_13824434_3364.cpp:70:19: error: increment of member 'Node::num' in read-only object
     else (it->num)++;
                   ^