diff --git a/src/lib.rs b/src/lib.rs
index 8910d72d..6fa4db48 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1345,6 +1345,7 @@ pub mod problem_1631_path_with_minimum_effort;
 pub mod problem_1632_rank_transform_of_a_matrix;
 pub mod problem_1636_sort_array_by_increasing_frequency;
 pub mod problem_1637_widest_vertical_area_between_two_points_containing_no_points;
+pub mod problem_1638_count_substrings_that_differ_by_one_character;
 pub mod problem_1640_check_array_formation_through_concatenation;
 pub mod problem_1641_count_sorted_vowel_strings;
 pub mod problem_1642_furthest_building_you_can_reach;
diff --git a/src/problem_1638_count_substrings_that_differ_by_one_character/iterative.rs b/src/problem_1638_count_substrings_that_differ_by_one_character/iterative.rs
new file mode 100644
index 00000000..220b2be7
--- /dev/null
+++ b/src/problem_1638_count_substrings_that_differ_by_one_character/iterative.rs
@@ -0,0 +1,55 @@
+pub struct Solution;
+
+// ------------------------------------------------------ snip ------------------------------------------------------ //
+
+// See <https://leetcode.com/problems/count-substrings-that-differ-by-one-character/discuss/917985/JavaC%2B%2BPython-Time-O(nm)-Space-O(1)>.
+
+impl Solution {
+    fn helper(s: &[u8], t: &[u8], result: &mut i32) {
+        let mut length_1 = 0;
+        let mut length_2 = 0;
+
+        for (left, right) in s.iter().zip(t) {
+            length_2 += 1;
+
+            if left != right {
+                length_1 = length_2;
+                length_2 = 0;
+            }
+
+            *result += length_1;
+        }
+    }
+
+    pub fn count_substrings(s: String, t: String) -> i32 {
+        let s = s.as_bytes();
+        let t = t.as_bytes();
+        let mut result = 0;
+
+        for i in 0..t.len() {
+            Self::helper(s, &t[i..], &mut result);
+        }
+
+        for i in 1..s.len() {
+            Self::helper(&s[i..], t, &mut result);
+        }
+
+        result
+    }
+}
+
+// ------------------------------------------------------ snip ------------------------------------------------------ //
+
+impl super::Solution for Solution {
+    fn count_substrings(s: String, t: String) -> i32 {
+        Self::count_substrings(s, t)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn test_solution() {
+        super::super::tests::run::<super::Solution>();
+    }
+}
diff --git a/src/problem_1638_count_substrings_that_differ_by_one_character/mod.rs b/src/problem_1638_count_substrings_that_differ_by_one_character/mod.rs
new file mode 100644
index 00000000..d87fb17c
--- /dev/null
+++ b/src/problem_1638_count_substrings_that_differ_by_one_character/mod.rs
@@ -0,0 +1,18 @@
+pub mod iterative;
+
+pub trait Solution {
+    fn count_substrings(s: String, t: String) -> i32;
+}
+
+#[cfg(test)]
+mod tests {
+    use super::Solution;
+
+    pub fn run<S: Solution>() {
+        let test_cases = [(("aba", "baba"), 6), (("ab", "bb"), 3)];
+
+        for ((s, t), expected) in test_cases {
+            assert_eq!(S::count_substrings(s.to_string(), t.to_string()), expected);
+        }
+    }
+}