# 101. Symmetric Tree (Easy)

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

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

For example, this binary tree `[1,2,2,3,4,4,3]` is symmetric:

```    1
/ \
2   2
/ \ / \
3  4 4  3
```

But the following `[1,2,2,null,3,null,3]` is not:

```    1
/ \
2   2
\   \
3    3
```

Note:
Bonus points if you could solve it both recursively and iteratively.

## Solutions

``````class Solution {
public boolean isSymmetric(TreeNode root) {
if (root == null) {
return true;
}

return check(root.left, root.right);
}

private boolean check(TreeNode root1, TreeNode root2) {
if (root1 == null && root2 == null) {
return true;
}

boolean c1 = root1 == null && root2 != null;
boolean c2 = root1 != null && root2 == null;

if (c1 || c2 || root1.val != root2.val) {
return false;
}

// to reach here, root1.val == root2.val

// check out corresponding branches, be aware that we compare root.left against
// root.right and root.right against root.left, rather than the same side branches
// comparison.

c1 = check(root1.left, root2.right);
c2 = check(root1.right, root2.left);

// only if both branches are symmetric, return true
return c1 && c2;
}
}
``````