View Compilation Error

0_0_27725685_6095.cpp: In member function 'void node::adopt(node*, int)':
0_0_27725685_6095.cpp:24:12: error: 'null' was not declared in this scope
   if (s != null)
            ^
0_0_27725685_6095.cpp: In member function 'void node::up()':
0_0_27725685_6095.cpp:35:16: error: 'null' was not declared in this scope
   if (ch[0] != null)
                ^
0_0_27725685_6095.cpp:37:16: error: 'null' was not declared in this scope
   if (ch[1] != null)
                ^
0_0_27725685_6095.cpp: In member function 'void node::down()':
0_0_27725685_6095.cpp:48:17: error: 'null' was not declared in this scope
    if (ch[0] != null)
                 ^
0_0_27725685_6095.cpp:50:17: error: 'null' was not declared in this scope
    if (ch[1] != null)
                 ^
0_0_27725685_6095.cpp:56:17: error: 'null' was not declared in this scope
    if (ch[0] != null)
                 ^
0_0_27725685_6095.cpp:58:17: error: 'null' was not declared in this scope
    if (ch[1] != null)
                 ^