delete test pass.

Author: happy-san

lib/trees/threaded_binary_tree.dart

@@ -166,13 +166,6 @@ class ThreadedBinaryTree<V extends Comparable>
     return result;
   }
 
-  void _postOrder(ThreadedBinaryNode<V>? node, List<V> list) {
-    if (node == null) return;
-    if (!(node.leftIsThread ?? false)) _postOrder(node.left, list);
-    if (!(node.rightIsThread ?? false)) _postOrder(node.right, list);
-    list.add(node.value!);
-  }
-
   @override
   List<V> preOrder() {
     var result = <V>[];
@@ -214,7 +207,7 @@ class ThreadedBinaryTree<V extends Comparable>
         break;
 
       case _DeleteCase.oneChild:
-        var child = node.leftIsThread! ? node.right : node.left;
+        var child = (node.leftIsThread ?? true) ? node.right : node.left;
 
         if (parent == null) {
           root = child;
@@ -228,17 +221,19 @@ class ThreadedBinaryTree<V extends Comparable>
         var predecessor = _inOrderPredecessor(node);
         if (!(node.leftIsThread ?? true)) {
           predecessor!.right = successor;
+          predecessor.rightIsThread = successor == null ? null : true;
         } else {
           if (!(node.rightIsThread ?? true)) {
             successor!.left = predecessor;
+            successor.leftIsThread = predecessor == null ? null : true;
           }
         }
         break;
 
       case _DeleteCase.twoChildren:
         var successor = node.right!, parentSuccessor = node;
 
-        while (successor.left != null && !successor.leftIsThread!) {
+        while (!successor.leftIsThread!) {
           parentSuccessor = successor;
           successor = successor.left!;
         }
@@ -279,6 +274,14 @@ class ThreadedBinaryTree<V extends Comparable>
       return node;
     }
   }
+
+  void _postOrder(ThreadedBinaryNode<V>? node, List<V> list) {
+    if (node == null) return;
+
+    if (!(node.leftIsThread ?? false)) _postOrder(node.left, list);
+    if (!(node.rightIsThread ?? false)) _postOrder(node.right, list);
+    list.add(node.value!);
+  }
 }
 
 enum _DeleteCase { twoChildren, oneChild, childless }