57.翻转二叉树

给你一棵二叉树的根节点 root ，翻转这棵二叉树，并返回其根节点。

/*解题思路
交换左子树和右子树，同时维护一个 temp 指针指向被破坏的 root.left，随后 root.right = invertTree(temp) 即可。否则交换会发生错误。

*/
/**
 * 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 invertTree(TreeNode root) {
        if (root == null) {
            return null;
        }
        TreeNode temp = root.left; 
        root.left = invertTree(root.right);
        root.right = invertTree(temp);
        return root;
    }
}