翻轉一棵二叉樹。

來源：力扣（LeetCode）
連結：https://leetcode-cn.com/problems/invert-binary-tree
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class Solution {

    private TreeNode reverse(TreeNode root) {
        if (root == null) {
            return root;
        }

        TreeNode left = root.left;
        root.left = reverse(root.right);
        root.right = reverse(left);

        return root;
    }

    public TreeNode invertTree(TreeNode root) {
        if (root == null) {
            return root;
        }
        return reverse(root);
    }
}

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;
    }
}