156. Binary Tree Upside Down

// Definition for a binary tree node.
class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;

    TreeNode(int x) {
        val = x;
    }
}

/**
 * Recursive Solution
 *
 * Time Complexity: O(N)
 *
 * Space Complexity: O(N)
 *
 * N = Number nodes in the tree or the height of the tree.
 */
class Solution1 {
    public TreeNode upsideDownBinaryTree(TreeNode root) {
        if (root == null || root.left == null) {
            return root;
        }

        TreeNode newRoot = upsideDownBinaryTree(root.left);
        root.left.left = root.right;
        root.left.right = root;
        root.left = null;
        root.right = null;

        return newRoot;
    }
}

/**
 * Iterative Solution
 *
 * Time Complexity: O(N)
 *
 * Space Complexity: O(1)
 *
 * N = Number nodes in the tree or the height of the tree.
 */
class Solution2 {
    public TreeNode upsideDownBinaryTree(TreeNode root) {
        if (root == null || root.left == null) {
            return root;
        }

        TreeNode cur = root;
        TreeNode pre = null;
        TreeNode preRight = null;

        while (cur != null) {
            TreeNode next = cur.left;

            cur.left = preRight;
            preRight = cur.right;

            cur.right = pre;

            pre = cur;
            cur = next;
        }

        return pre;
    }
}