979. Distribute Coins in Binary Tree

    var move = 0
    func distributeCoins(_ root: TreeNode?) -> Int {
        helper(root)
        return move
    }

    func helper( _ root: TreeNode?) -> (Int, Int){
        guard let root = root else { return ( 0 , 0 )}
        let left = helper(root.left)
        let right = helper(root.right)
        move += abs( left.0 - left.1) + abs(right.0 - right.1)
        return (left.0 + right.0 + 1 , left.1 + right.1 + root.val)
    }