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public TreeNode head = null;//双向链表的头节点
public TreeNode cur = null;//双向链表的尾节点游标

public TreeNode Convert(TreeNode pRootOfTree) {
if(pRootOfTree==null){
return null;
}

Convert(pRootOfTree.left);

if(cur == null){
head = pRootOfTree;
cur = pRootOfTree;
}else{
cur.right = pRootOfTree;
pRootOfTree.left = cur;
cur = pRootOfTree;
}

Convert(pRootOfTree.right);
return head;
}

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if(cur为空) → 初始化链表，头和尾都是第一个节点；
else → 把当前节点拼到链表尾部，完成双向绑定，然后移动尾指针。