对称二叉树

题目描述：
给定一个二叉树，检查它是否是镜像对称的。

示例：
例如，二叉树 [1,2,2,3,4,4,3] 是对称的。
1
/ \
2 2
/ \ / \
3 4 4 3

Java代码：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    //递归
    public boolean isSymmetric(TreeNode root) {
        return isMirror(root, root);
    }

    public boolean isMirror(TreeNode t1, TreeNode t2) {
        if(t1 == null && t2 == null) return true;
        if(t1 == null || t2 == null) return false;

        return (t1.val == t2.val) && isMirror(t1.left, t2.right) && isMirror(t1.right, t2.left); 
    }

    //迭代
    public boolean isSymmetric(TreeNode root) {
        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        queue.add(root);

        while(!queue.isEmpty()) {
            TreeNode t1 = queue.poll();
            TreeNode t2 = queue.poll();
            if(t1 == null && t2 == null) continue;
            if(t1 == null || t2 == null) return false;
            if(t1.val != t2.val) return false;
            queue.add(t1.left);
            queue.add(t2.right);
            queue.add(t1.right);
            queue.add(t2.left);
        }

        return true;
    }
}