0101.java

/**
 * 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 isMirror(root.left, root.right);
    }
    
    boolean isMirror (TreeNode t1, TreeNode t2){
        if (t1==null && t2==null) return true;
        if (t1==null || t2==null) return false;
        if (t1.val != t2.val) return false;
        return isMirror(t1.left, t2.right) && isMirror(t1.right, t2.left);
    }
}