Added custom sort string solution.

README.md

@@ -147,6 +147,7 @@ other languages.
 - Sort Colors (https://leetcode.com/problems/sort-colors)
 - Rotate Digits (https://leetcode.com/problems/rotated-digits)
 - Sort Jumbled Numbers (https://leetcode.com/problems/sort-the-jumbled-numbers)
+- Custom Sort String (https://leetcode.com/problems/custom-sort-string)
 
 ### Other
 

src/leetcode/custom_sort.rs

@@ -0,0 +1,24 @@
+pub fn custom_sort_string(order: String, mut s: String) -> String {
+    let order = order.char_indices().fold([0; 26], |mut acc, (idx, c)| {
+        acc[c as usize - 'a' as usize] = idx;
+        acc
+    });
+    unsafe {
+        s.as_mut_vec()
+            .sort_by_key(|c| order[*c as usize - 'a' as usize])
+    };
+    s
+}
+
+#[cfg(test)]
+mod tests {
+    use super::custom_sort_string;
+
+    #[test]
+    fn sorts_in_custom_order() {
+        let order = "cba".to_string();
+        let s = "abc".to_string();
+        let result = custom_sort_string(order, s);
+        assert_eq!("cba".to_string(), result)
+    }
+}

src/leetcode/mod.rs

@@ -50,3 +50,4 @@ mod max_repeating_substr;
 mod freq_sort;
 mod sort_jumbled_numbers;
 mod max_binary_number;
+mod custom_sort;