View Compilation Error

0_0_4822373_13465.cpp:22: error: `NULL' was not declared in this scope
0_0_4822373_13465.cpp: In constructor `LinkStack<DataType>::node::node()':
0_0_4822373_13465.cpp:21: error: `NULL' undeclared (first use this function)
0_0_4822373_13465.cpp:21: error: (Each undeclared identifier is reported only once for each function it appears in.)
0_0_4822373_13465.cpp: In member function `bool LinkStack<DataType>::empty()':
0_0_4822373_13465.cpp:37: error: `NULL' undeclared (first use this function)
0_0_4822373_13465.cpp: In destructor `LinkStack<DataType>::~LinkStack()':
0_0_4822373_13465.cpp:69: error: `NULL' undeclared (first use this function)