LeetCode_101 Symmetric Tree

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).

For example, this binary tree is symmetric:

1

/ \

2 2

/ \ / \

3 4 4 3

But the following is not:

1

/ \

2 2

\ \

3 3

public boolean isSymmetric(TreeNode root) {
if (root ==null) return true;
return symmetric(root.left, root.right);
}
private boolean symmetric(TreeNode left, TreeNode right){
if (left == null && right ==null) return true;
if (left !=null && right !=null &&right.val == left.val) {
return symmetric(left.right, right.left)&&symmetric(left.left, right.right);
}else {
return false;
}
}