What are expression trees? 

The leaves of an expression tree are operands, like as constants or variable names, and the other nodes have operators. This certain tree happens to be binary, because all of the operations are binary, and although this is the easiest case, it is possible for nodes to have more than two children. It is also possible for a node to have only single child, as is the case with the unary minus operator. We can evaluate an expression tree, T, by applying the operator at the root to the values obtain by recursively evaluating the left and right subtrees.