假如有父进程 A 和 子进程 B、C、D，还有 B 的子进程 B1，B2

• A -> children -> next = B -> sibling
• B -> sibling -> next = C -> sibling
• C -> sibling -> next = D -> sibling
• D -> sibling -> next = A -> children
• B -> children -> next = B1 -> sibling
• B1 -> sibling -> next = B2 -> sibling
• B2 -> sibling -> next = B -> children