/**
 * 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 MergeTwoBinaryTrees {
    func mergeTrees(_ t1: TreeNode?, _ t2: TreeNode?) -> TreeNode? {
        guard let t1 = t1 else {
            return t2
        }
        guard let t2 = t2 else {
            return t1
        }
        t1.val += t2.val
        t1.left = mergeTrees(t1.left, t2.left)
        t1.right = mergeTrees(t1.right, t2.right)
        return t1
    }
}