递归法：

/**
 * 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 boolean isSymmetric(TreeNode root) {
        if(root==null)return true;
        return compare(root.left,root.right);
    }
    public boolean compare(TreeNode left,TreeNode right){
        if(left==null&&right!=null)return false;//左空右不空，非对称
        else if(left!=null&&right==null)return false;//右空左不空，非对称
        else if(left==null&&right==null)return true;//左空右空，对称
        else if(left.val!=right.val)return false;//左值≠右值，非对称
        
        boolean outside=compare(left.left,right.right);
        boolean inside=compare(left.right,right.left);
        boolean isSame=outside&&inside;
        return isSame;
    }
}