Added single number II solution

README.md

@@ -208,7 +208,8 @@ programming challenges completed in other languages.
 - [Take K Characters From Left and Right](https://leetcode.com/problems/take-k-of-each-character-from-left-and-right)
 - [Unguarded Cells](https://leetcode.com/problems/count-unguarded-cells-in-the-grid)
 - [Rotate the Box](https://leetcode.com/problems/rotating-the-box)
-- [Find Champion](https://leetcode.com/problems/find-champion-ii/)
+- [Find Champion II](https://leetcode.com/problems/find-champion-ii/)
+- [Single Number II](https://leetcode.com/problems/find-champion-ii/)
 
 #### Hard
 

src/leetcode/mod.rs

@@ -115,3 +115,4 @@ mod make_increasing;
 mod rotate_box;
 mod find_champion;
 mod single_number;
+mod single_number_ii;

src/leetcode/single_number_ii.rs

@@ -0,0 +1,27 @@
+use std::collections::HashMap;
+
+pub fn single_number(numbers: Vec<i32>) -> i32 {
+    let counter = numbers.into_iter().fold(HashMap::new(), |mut acc, curr| {
+        *acc.entry(curr).or_insert(0) += 1;
+        acc
+    });
+    counter.into_iter().find(|(_, v)| *v == 1).unwrap().0
+}
+
+// Optimized solution
+// Credit: https://leetcode.com/problems/single-number-ii/solutions/2389678/rust-concise-solution-with-ternary-logic-explained/
+// pub fn single_number(numbers: Vec<i32>) -> i32 {
+//     numbers
+//         .into_iter()
+//         .fold([0, 0], |[a, b], x| [a ^ x & !b, a & x | b & !x])[0]
+// }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn finds_the_number_that_appears_once() {
+        assert_eq!(3, single_number(vec![2, 2, 3, 2]));
+    }
+}