Solution.java

public class Solution {
    //从上向下递归，检验镜像树
    public boolean isSymmetric(TreeNode root) {
        if(null == root){
            return true;
        }
        return isMirror(root.left, root.right);
    }

    private boolean isMirror(TreeNode left, TreeNode right){
        if(left == null && right == null) return true;
        if(left == null || right == null) return false;
        return (left.val == right.val) 
        && isMirror(left.left, right.right) 
        && isMirror(left.right, right.left) ;
    }
}