Solve missing-number.

source/lib.rs

@@ -2,6 +2,7 @@ pub mod add_binary;
 pub mod implement_strstr;
 pub mod length_of_last_word;
 pub mod longest_common_prefix;
+pub mod missing_number;
 pub mod palindrome_number;
 pub mod plus_one;
 pub mod reverse_integer;

source/missing_number/mod.rs

@@ -0,0 +1,18 @@
+pub fn missing_number(numbers: Vec<i32>) -> i32 {
+  // The range of numbers can be described as the total sum of all possible
+  // numbers in that range.
+  let total_range = (0..=numbers.len() as i32).sum::<i32>();
+
+  // And since only one number is missing from that range, the difference
+  // between the range's sum and the numbers' sum will be that missing number.
+  let sum_of_numbers = numbers.iter().sum::<i32>();
+
+  total_range - sum_of_numbers
+}
+
+#[test]
+fn test_missing_number() {
+  assert_eq!(missing_number(vec![3, 0, 1]), 2);
+  assert_eq!(missing_number(vec![0, 1]), 2);
+  assert_eq!(missing_number(vec![9, 6, 4, 2, 3, 5, 7, 0, 1]), 8);
+}