二叉搜索树转化成双向链表

/**
public class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;

    public TreeNode(int val) {
        this.val = val;

    }

}
*/
public class Solution {
    private TreeNode pre = null; // 因为没有二级指针，只能用全局变量了
    public TreeNode Convert(TreeNode pRootOfTree) {
        if (pRootOfTree == null)    return null;
        if (pRootOfTree.left == null && pRootOfTree.right == null)    return pRootOfTree;
        LinkedNode(pRootOfTree);
        TreeNode cur = pRootOfTree;
        while (cur.left != null) {
            cur = cur.left;
        }
        return cur;
    }
    private void LinkedNode(TreeNode root) {
        if (root.left != null) {
            LinkedNode(root.left);
        }
        root.left = pre;
        if (pre != null)    pre.right = root;
        pre = root;
        if (root.right != null) {
            LinkedNode(root.right);
        }
    }
}