# 101. Symmetric Tree

<https://leetcode.com/problems/symmetric-tree/>

## solution

* 关于递归的返回。注释掉的部分，会导致结果错误，甚至没有继续向下判断

```python
class Solution:
    def isSymmetric(self, root: Optional[TreeNode]) -> bool:
        if root is None:
            return True
        return self.isSym(root.left, root.right)

    def isSym(self, l, r):
        if l is None and r is not None:
            return False
        elif l is not None and r is None:
            return False
        elif l is None and r is None:
            return True

        if l.val != r.val:
            return False
        # else:
        #     return True

        return self.isSym(l.left, r.right) and self.isSym(l.right, r.left)
```

时间复杂度：O(n)\
空间复杂度：O(h)
