Evaluating expression trees

Question

Skiena's book on Algorithm contains the following question:

1) Evaluate expression given as binary tree in O(n) time, given n nodes.

2) Evaluate expression given as DAG in O(n+m) time, given n nodes and m edges in DAG.

I could think of a way for the first question:

evaluate(node) {
     if(node has children){
          left_val = evaluate(node->left);
          right_val = evaluate(node->right);

          // find operation symbol for node and use it as
          // val = left_val operation right_val

          return val;
     }
     else {
          return node_value;
     }
}

Since we visit each node once, it will take O(n) time.

Since the book has no solutions, can anyone please tell if this is correct ?

Also can anyone suggest a solution for second question.

Thanks.

Answer 1

First way looks fine to me.

For the DAG, if you can modify the tree to add cached values to each node, you can use the same algorithm with a small tweak to not recurse if an operator node has a cached value. This should be O(n+m) time (at most one arithmetic operation per node and at most one pointer lookup per edge). Explicitly:

evaluate(node) {
     if (node has value) {
          return node->value;
     } else {
          left = evaluate(node->left);
          right = evaluate(node->right);

          // find operation symbol for node and use it as
          // val = left_val operation right_val

          node->value = val;
          return val;
     }
}