897. 递增顺序搜索树

给你一棵二叉搜索树，请你 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。

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/increasing-order-search-tree
著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

class Solution {

    private TreeNode[] solve(TreeNode root) {
        if (root == null) {
            return new TreeNode[]{null, null};
        }
        TreeNode[] left = solve(root.left);
        TreeNode[] right = solve(root.right);
        TreeNode[] ans = new TreeNode[]{root, root};
        if (left[0] != null) {
            ans[0] = left[0];
            left[1].right = root;
        }

        if (right[0] != null) {
            ans[1] = right[1];
            root.right = right[0];
        }
        root.left = null;
        return ans;
    }

    public TreeNode increasingBST(TreeNode root) {
        return solve(root)[0];
    }
}

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;
    }
}