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.
/**
* Definition for a binary tree node.
* public class TreeNode {
* public var val: Int
* public var left: TreeNode?
* public var right: TreeNode?
* public init(_ val: Int) {
* self.val = val
* self.left = nil
* self.right = nil
* }
* }
*/
class Solution {
func isSymmetric(_ root: TreeNode?) -> Bool {
func isSymmetric(left: TreeNode?, right: TreeNode?) -> Bool {
guard let l = left, let r = right, l.val == r.val else {
return (left == nil && right == nil)
}
return isSymmetric(left: l.left, right: r.right) && isSymmetric(left: l.right, right: r.left)
}
return isSymmetric(left: root?.left, right: root?.right)
}
}