Added binary search solution.

README.md

@@ -163,6 +163,7 @@ programming challenges completed in other languages.
 - [Final Prices After Discounts](https://leetcode.com/problems/final-prices-with-a-special-discount-in-a-shop)
 - [Valid Palindrome](https://leetcode.com/problems/valid-palindrome)
 - [Roman to Integer](https://leetcode.com/problems/roman-to-integer)
+- [Binary Search](https://leetcode.com/problems/binary-search)
 
 #### Medium
 

src/leetcode/binary_search.rs

@@ -0,0 +1,21 @@
+pub fn search(numbers: Vec<i32>, target: i32) -> i32 {
+    match numbers.binary_search(&target) {
+        Ok(i) => i as i32,
+        Err(_) => -1,
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn finds_element_index_when_it_exists() {
+        assert_eq!(4, search(vec![-1, 0, 3, 5, 9, 12], 9))
+    }
+
+    #[test]
+    fn negative_one_when_element_does_not_exist() {
+        assert_eq!(-1, search(vec![-1, 0, 3, 5, 9, 12], 20))
+    }
+}

src/leetcode/mod.rs

@@ -151,3 +151,4 @@ mod xor_subarray;
 mod zero_or_one;
 mod zigzag_conversion;
 mod valid_sudoku;
+mod binary_search;