NodeBuilder<V> addChild(NodeBuilder<V> child) {
            requireNonNull(child, "child");

            final char key = child.extractKey();
            if (children == null) {
                children = new Char2ObjectOpenHashMap<>();
            }
            if (children.containsKey(key)) {
                // There should not exist two different children which starts with the same character in a trie.
                throw new IllegalStateException("Path starting with '" + key + "' already exist:" + child);
            }
            children.put(key, child);
            return child;
        }
Adds a child {@link RoutingTrie.Node} into the {@code children} map.
core/src/main/java/com/linecorp/armeria/server/RoutingTrieBuilder.java