Что такое устойчивые узлы?

---

What is a sustabable nodes?

У меня есть узлы g (зеленый) и b (черный.
Вот вам поручение:

This exercise is about rooted binary trees. Each node x has fields x.parent, x.left and x.right denoting the parent, left
child, and right child of x. The root of the tree is denoted by root and root.parent = null. Furthermore, each node x also
has a field x.color, called the color of x, that can be either green or black. Throughout the exercise, we let n denote the
size of the input tree.

2.1 (2 %) A node x is sustainable if the color of x is green and the color of all descendants of x is also green. Consider
the tree below. Nodes with a g are green and nodes with a b are black. Mark all sustainable nodes in the tree. Do so
directly on the vertices in the tree.

Что я уже пробовал:

Я никогда не слышал об устойчивых узлах, как это должно работать?

---