226. 翻转二叉树

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典型的递归题
先递归到左叶子节点，再递归到右叶子节点，然后再交换，直接上图

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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode invertTree(TreeNode root) {
        if(root==null||(root.left==null&&root.right==null)) return root;
        TreeNode left=invertTree(root.left);
        TreeNode right=invertTree(root.right);
        root.left=right;
        root.right=left;
        return root;
    }
}