给你二叉搜索树的根节点 root ，该树中的两个节点的值被错误地交换。请在不改变其结构的情况下，恢复这棵树。

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/recover-binary-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 {
    public void recoverTree(TreeNode root) {
        if (root == null) {
            return;
        }
        TreeNode first = null, second = null;
        TreeNode pre = null, cur = root;
        while (cur != null) {
            TreeNode mostRight = cur.left;
            if (mostRight != null) {
                while (mostRight.right != null && mostRight.right != cur) {
                    mostRight = mostRight.right;
                }
                if (mostRight.right == null) {
                    mostRight.right = cur;
                    cur = cur.left;
                    continue;
                } else {
                    mostRight.right = null;
                }
            }
            if (pre != null && pre.val > cur.val) {
                if (first == null) {
                    first = pre;
                    second = cur;
                } else {
                    second = cur;
                }
            }
            pre = cur;
            cur = cur.right;
        }
        swap(first, second);
    }

    public void swap(TreeNode a, TreeNode b) {
        int t = a.val;
        a.val = b.val;
        b.val = t;
    }
}

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;
    }
}