diff --git a/README.md b/README.md
index c975348..702bd0c 100644
--- a/README.md
+++ b/README.md
@@ -121,6 +121,7 @@ continually updating 😃.
 * [13. Roman to Integer](src/0013_roman_to_integer/roman_to_integer.go)   *`string`*
 * [66. Plus One](src/0066_plus_one/plus_one.go)   *`array`*
 * [150. Evaluate Reverse Polish Notation](src/0150_evaluate_reverse_polish_notation/evaluate_reverse_polish_notation.go)   *`stack`*
+* [258. Add Digits](src/0258_add_digits/add_digits.go)
 
 <details>
 </details>
diff --git a/src/0258_add_digits/add_digits.go b/src/0258_add_digits/add_digits.go
new file mode 100644
index 0000000..246599c
--- /dev/null
+++ b/src/0258_add_digits/add_digits.go
@@ -0,0 +1,41 @@
+/*
+258. Add Digits
+https://leetcode.com/problems/add-digits/
+
+Given a non-negative integer num, repeatedly add all its digits until the result has only one digit.
+*/
+// time: 2019-01-07
+
+package ad
+
+// time complexity: O(	log num	)
+// space complexity: O(1)
+func addDigits(num int) int {
+	// num is a non-negative integer
+	for num > 9 {
+		num = performAdd(num)
+	}
+	return num
+}
+
+func performAdd(num int) (res int) {
+	for num > 0 {
+		res += num % 10
+		num /= 10
+	}
+	return
+}
+
+// time complexity: O(1)
+// space complexity: O(1)
+func addDigits1(num int) int {
+	/*
+		0	1	2	3	4	5	6	7	8		9
+		0	1	2	3	4	5	6	7	8		9
+		10	11	12	13	14	15	16	17	18		19
+		1	2	3	4	5	6	7	8	9		10/1
+		20	21	22	23	24	25	26	27	28		29
+		2	3	4	5	6	7	8	9	10/1		11/2
+	*/
+	return (num-1)%9 + 1
+}
diff --git a/src/0258_add_digits/add_digits_test.go b/src/0258_add_digits/add_digits_test.go
new file mode 100644
index 0000000..58aaa10
--- /dev/null
+++ b/src/0258_add_digits/add_digits_test.go
@@ -0,0 +1,19 @@
+package ad
+
+import "testing"
+
+func TestAddDigits(t *testing.T) {
+	num := 38
+	expected := 2
+
+	testFuncs := []func(int) int{
+		addDigits,
+		addDigits1,
+	}
+
+	for _, function := range testFuncs {
+		if res := function(num); res != expected {
+			t.Errorf("expected %d, got %d", expected, res)
+		}
+	}
+}
diff --git a/src/README.md b/src/README.md
index 0cbd1e7..38749dc 100644
--- a/src/README.md
+++ b/src/README.md
@@ -74,6 +74,7 @@
 |0235|[235. Lowest Common Ancestor of a Binary Search Tree](0235_lowest_common_ancestor_of_a_binary_search_tree/lcaoabst.go)|Easy|*`recursion; `* *`binary tree`*|
 |0237|[237. Delete Node in a Linked List](0237_delete_node_in_a_linked_list/dniall.go)|Easy|*`linked list`*|
 |0257|[257. Binary Tree Paths](0257_binary_tree_paths/binary_tree_paths.go)|Easy|*`binary tree`*|
+|0258|[258. Add Digits](0258_add_digits/add_digits.go)|Easy|*`math`*|
 |0283|[Move Zeroes(solution1)](./0283_move_zeroes/move_zeroes.go) <br/>  [Move Zeroes(solution2)](./0283_move_zeroes/move_zeroes2.go)|Easy|*`array`*|
 |0300|[Longest Increasing Subsequence](./0300_longest_increasing_subsequence/lis.go)|Medium|*`dp`*|
 |0303|[303. Range Sum Query - Immutable](0303_range_sum_query/rsqim.go)|Easy||