[LeetCode]第十六题 ：对称树

题目描述：

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).

For example, this binary tree [1,2,2,3,4,4,3] is symmetric:

    1
   / \
  2   2
 / \ / \
3  4 4  3

But the following [1,2,2,null,3,null,3] is not:

    1
   / \
  2   2
   \   \
   3    3

Note:
Bonus points if you could solve it both recursively and iteratively.

题目解释：

给出一个二叉树，判断它是否是对称的。

题目解法：

1.我的解法。首先树肯定要用递归；判断左子树和右子树是否对称，先判断左子树的结构和右子树的结构是否相同，在判断值是否相同；最后递归调用即可。代码如下：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public boolean isSymmetric(TreeNode root) {
        if(root == null) return true;
        return a(root.left,root.right);
    }
    
    private boolean a(TreeNode left,TreeNode right) {
        if(left == null && right == null) return true;
        if((left == null && right != null) || (left != null && right == null)) return false;
        if(left.val != right.val) return false;
        return true && a(left.right,right.left) && a(left.left,right.right);
    } 
}