Question

How can I traverse an n-ary tree without using recursion?

Recursive way:

traverse(Node node)
{
    if(node == null)
        return;

    for(Node child : node.getChilds()) {
        traverse(child);
    }
}

Answer 1

You can do this without recursion and without a stack. But you need to add two extra pointers to the node:

1. The parent node. So you can come back to the parent if you are finished.

2. The current child node so you know which one to take next.

   • For each node, you handle all the kids.
   • If a kid is handled, you check if there is a next kid and handle that (updating the current).
   • If all kids are handled, go back to the parent.
   • If the node is NULL, quit.

With pseudocode this looks like:

traverse(Node node) {
  while (node) {
    if (node->current <= MAX_CHILD) {
      Node prev = node;
      if (node->child[node->current]) {
        node = node->child[node->current];
      }
      prev->current++;
    } else {
      // Do your thing with the node.
      node->current = 0; // Reset counter for next traversal.
      node = node->parent;
    }
  }
}