Added alternating digit sum solution.

README.md

@@ -122,6 +122,7 @@ other languages.
 - Sort by Increasing Frequency (https://leetcode.com/problems/sort-array-by-increasing-frequency)
 - Maximum Odd Binary Number (https://leetcode.com/problems/maximum-odd-binary-number)
 - Maximum Count of Positive or Negative Integer (https://leetcode.com/problems/maximum-count-of-positive-integer-and-negative-integer)
+- Alternating Digit Sum (https://leetcode.com/problems/alternating-digit-sum/)
 
 #### Medium
 

src/leetcode/alternating_digit_sum.rs

@@ -0,0 +1,24 @@
+pub fn alternate_digit_sum(mut number: i32) -> i32 {
+    let mut result = 0;
+    let mut sign = 1;
+    while number > 0 {
+        sign *= -1;
+        result += (number % 10) * sign;
+        number /= 10;
+    }
+    result * sign
+}
+
+#[cfg(test)]
+mod tests {
+    use super::alternate_digit_sum;
+
+    #[test]
+    fn digits_are_summed_with_alternating_signs() {
+        let sum = alternate_digit_sum(12345);
+        assert_eq!(3, sum);
+
+        let sum = alternate_digit_sum(10);
+        assert_eq!(1, sum)
+    }
+}

src/leetcode/mod.rs

@@ -52,3 +52,4 @@ mod sort_jumbled_numbers;
 mod max_binary_number;
 mod custom_sort;
 mod max_positive_negative;
+mod alternating_digit_sum;