101. 对称二叉树

class Solution {
    public boolean isSymmetric(TreeNode root) {
        if (root==null){
            return true;
        }
        return dfs(root.left,root.right);
    }

    private boolean dfs(TreeNode left, TreeNode right) {
        //如果全null
        if (left==null&&right==null){
            return true;
        }
        //如果只有一个为null
        if (left==null||right==null){
            return false;
        }
        //如果均不为null
        if (left.val!=right.val){
            return false;
        }
        return dfs(left.left,right.right)&&dfs(left.right,right.left);
    }
}