View Compilation Error

0_0_4359495_10791.cpp: In function `treeNode* newNode()':
0_0_4359495_10791.cpp:26: error: `NULL' undeclared (first use this function)
0_0_4359495_10791.cpp:26: error: (Each undeclared identifier is reported only once for each function it appears in.)
0_0_4359495_10791.cpp: In function `treeNode* mulTerm()':
0_0_4359495_10791.cpp:79: error: `NULL' undeclared (first use this function)
0_0_4359495_10791.cpp: In function `void cal(treeNode*)':
0_0_4359495_10791.cpp:159: error: `pow' undeclared (first use this function)
0_0_4359495_10791.cpp: In function `int main()':
0_0_4359495_10791.cpp:185: error: `scanf' undeclared (first use this function)
0_0_4359495_10791.cpp:186: error: `getchar' undeclared (first use this function)
0_0_4359495_10791.cpp:188: error: `gets' undeclared (first use this function)
0_0_4359495_10791.cpp:190: error: `printf' undeclared (first use this function)