Leetcode #437: Path Sum iii

func pathSum(_ root: TreeNode?, _ sum: Int) -> Int {
    if root == nil {
        return 0
    }
    return helper(root, sum) + pathSum(root?.left, sum) + pathSum(root?.right, sum)
}

private func helper(_ node: TreeNode?, _ target: Int) -> Int {
    if node == nil {
        return 0
    }
    
    var result = 0
    if node!.val == target {
        result += 1
    }
    result += (helper(node?.left, target-node!.val) + helper(node?.right, target-node!.val))
    return result
}