Handle nils in traversal

Skip nil children during postorder and breadth-first traversals so binary
nodes with missing children do not break traversal. Add a regression test
covering nil child traversal paths.

Author: evolve75

lib/tree.rb

@@ -679,7 +679,7 @@ def postordered_each
           peek_node.visited = true
           # Add the children to the stack. Use the marking structure.
           marked_children =
-            peek_node.node.children.map { |node| marked_node.new(node, false) }
+            peek_node.node.children.compact.map { |node| marked_node.new(node, false) }
           node_stack = marked_children.concat(node_stack)
           next
         else
@@ -710,9 +710,11 @@ def breadth_each
       # Use a queue to do breadth traversal
       until node_queue.empty?
         node_to_traverse = node_queue.shift
+        next unless node_to_traverse
+
         yield node_to_traverse
         # Enqueue the children from left to right.
-        node_to_traverse.children { |child| node_queue.push child }
+        node_to_traverse.children { |child| node_queue.push child if child }
       end
 
       self if block_given?

test/test_binarytree.rb

@@ -309,5 +309,27 @@ def test_swap_children
       assert_equal(@right_child1, @root[0], 'right_child1 should now be the first child')
       assert_equal(@left_child1, @root[1], 'left_child1 should now be the last child')
     end
+
+    # Test traversals when nil children exist.
+    def test_traversal_with_nil_children
+      @root << @left_child1
+      @root << @right_child1
+
+      @root.right_child = nil
+
+      breadth_nodes = []
+      post_nodes = []
+
+      assert_nothing_raised do
+        @root.breadth_each { |node| breadth_nodes << node }
+      end
+
+      assert_nothing_raised do
+        @root.postordered_each { |node| post_nodes << node }
+      end
+
+      assert_equal([@root, @left_child1], breadth_nodes)
+      assert_equal([@left_child1, @root], post_nodes)
+    end
   end
 end