BST转成双向链表

public class Solution {
    public TreeNode Convert(TreeNode pRootOfTree) {
        // 根节点为空
        if(pRootOfTree == null) return null;

        if(pRootOfTree.left != null){
            // 先转换左子树，获得转换后的头指针
            TreeNode left = Convert(pRootOfTree.left);

            // 获得指向左子树的最后一个元素的指针
            while(left.right != null) left = left.right;

            // 与root链接，注意双向
            pRootOfTree.left = left;
            left.right = pRootOfTree;
        }

        if(pRootOfTree.right != null){
            // 同理，转换右子树
            TreeNode right = Convert(pRootOfTree.right);

            // 获得指向右子树的第一个元素的指针，这里直接就是right，与root双向链接
            pRootOfTree.right = right;
            right.left = pRootOfTree;
        }

        // 最后把pRootOfTree指向双向链表的第一个元素再返回
        while(pRootOfTree.left != null){
            pRootOfTree = pRootOfTree.left;
        }

        return pRootOfTree;
    }
}