offer2-52

给你一棵二叉搜索树，请 按中序遍历 将其重新排列为一棵递增顺序搜索树，使树中最左边的节点成为树的根节点，并且每个节点没有左子节点，只有一个右子节点。

示例 1：

输入：root = [5,3,6,2,4,null,8,1,null,null,null,7,9]
输出：[1,null,2,null,3,null,4,null,5,null,6,null,7,null,8,null,9]

示例 2：

输入：root = [5,1,7]
输出：[1,null,5,null,7]

提示：

• 树中节点数的取值范围是 [1, 100]
• 0 <= Node.val <= 1000

//leetcode submit region begin(Prohibit modification and deletion)
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public TreeNode increasingBST(TreeNode root) {
        TreeNode head = root;
        while (head.left != null) {
            head = head.left;
        }
        doSort(root);
        return head;
    }



    private TreeNode doSort(TreeNode node) {
        if (node == null) {
            return node;
        }

        if (node.left != null) {
            TreeNode leftLast = doSort(node.left);
            leftLast.left = null;
            leftLast.right = node;
        }
        node.left = null;
        if (node.right != null) {
            TreeNode rightFirst = node.right;
            while (rightFirst.left != null) {
                rightFirst = rightFirst.left;
            }
            TreeNode rightLast = doSort(node.right);
            node.right = rightFirst;
            return rightLast;
        } else {
            return node;
        }
    }
}
//leetcode submit region end(Prohibit modification and deletion)