original express :  (1+2)
after building the tree :  (1+2)
evaluate the tree, the value should be :  3 result is :  3
original express :  ((3+4)*(5-6))
after building the tree :  ((3+4)*(5-6))
evaluate the tree, the value should be :  -7 result is :  -7
original express :  (2*(3+4))
after building the tree :  (2*(3+4))
evaluate the tree, the value should be :  14 result is :  14
original express :  ((2+3)*4))
after building the tree :  ((2+3)*4)
evaluate the tree, the value should be :  20 result is :  20
original express :  (((5+3)*4)+(2*6))
after building the tree :  (((5+3)*4)+(2*6))
evaluate the tree, the value should be :  44 result is :  44
original express :  ((a+x)*y)
after building the tree :  ((a+x)*y)
evaluate the tree, the value should be :  70 result is :  70