Added sort by increasing frequency solution.

README.md

@@ -119,6 +119,7 @@ other languages.
 - Remove From Sorted Array (https://leetcode.com/problems/remove-duplicates-from-sorted-array/)
 - Sort the People (https://leetcode.com/problems/sort-the-people)
 - Max Repeating Substring (https://leetcode.com/problems/maximum-repeating-substring/)
+- Sort by Increasing Frequency (https://leetcode.com/problems/sort-array-by-increasing-frequency)
 
 #### Medium
 

src/leetcode/freq_sort.rs

@@ -0,0 +1,23 @@
+use std::collections::HashMap;
+
+pub fn frequency_sort(mut numbers: Vec<i32>) -> Vec<i32> {
+    let mut counter: HashMap<i32, i32> = HashMap::new();
+    for x in numbers.iter() {
+        *counter.entry(*x).or_default() += 1;
+    }
+
+    numbers.sort_by_key(|n| (counter.get(n).unwrap(), -n));
+    numbers
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn items_sorted_by_frequency() {
+        let numbers = vec![2, 3, 1, 3, 2];
+        let actual = frequency_sort(numbers);
+        assert_eq!(vec![1, 3, 3, 2, 2], actual)
+    }
+}

src/leetcode/mod.rs

@@ -47,3 +47,4 @@ mod x_kind;
 mod remove_duplicates;
 mod sort_people;
 mod max_repeating_substr;
+mod freq_sort;