Suppose an arithmetic expression is given as a DAG (directed acyclic graph) with common subexpressions removed. Each leaf is an integer and each internal node is one of the standard arithmetical operations (+, -, *, /). For example, the expression 2 + 3 * 4 + (3 * 4)/5 is represented by the DAG in Figure (see book)(b). Give an O(n + m) algorithm for evaluating such a DAG, where there are n nodes and m edges in the DAG