Program received signal SIGSEGV, Segmentation fault.
0x00000000004012b9 in ExpressionTree::createTree (this=0x7fffffffddb0, str="*") at ExpressionTree.h:81
81 t1 = st.top();
(gdb) backtrace
#0 0x00000000004012b9 in ExpressionTree::createTree (this=0x7fffffffddb0, str="*") at ExpressionTree.h:81