View Compilation Error

0_0_27289118_29560.cpp: In function 'bool isBalancedTree(AVLNode*)':
0_0_27289118_29560.cpp:10:14: error: 'NULL' was not declared in this scope
  if(pRoot == NULL)
              ^
0_0_27289118_29560.cpp:12:38: error: 'getDepth' was not declared in this scope
  int leftDepth = getDepth(pRoot->left)
                                      ^
0_0_27289118_29560.cpp: In function 'int getDepth(AVLNode*)':
0_0_27289118_29560.cpp:21:14: error: 'NULL' was not declared in this scope
  if(pRoot == NULL) 
              ^
0_0_27289118_29560.cpp:24:2: error: expected ',' or ';' before 'int'
  int rightDepth = getDepth(pRoot->right)
  ^