diff --git a/src/euler51/src/main.rs b/src/euler51/src/main.rs
index f82733c..b565c21 100644
--- a/src/euler51/src/main.rs
+++ b/src/euler51/src/main.rs
@@ -34,14 +34,16 @@ fn main() {
     let all_primes = Primes::get_between(MIN_VALUE, MAX_VALUE);
     let mut processed_cases = HashSet::new();
     let mut primes_matching_pattern = Vec::with_capacity(TARGET_LENGTH);
+    let mut case = String::with_capacity(LENGTH);
 
     for pattern in PositionCombinations::new(MIN_LENGTH, MAX_LENGTH, LENGTH) {
         for prime in &all_primes.vector {
+            case.clear();
             primes_matching_pattern.clear();
 
             let digits = prime.to_digits();
 
-            let case = get_case_string(&digits, &pattern);
+            get_case_string(&digits, &pattern, &mut case);
             if processed_cases.contains(&case) {
                 continue;
             } else {
@@ -63,12 +65,12 @@ fn main() {
     }
 }
 
-fn get_case_string(digits: &[u8], pattern: &[bool]) -> String {
+fn get_case_string(digits: &[u8], pattern: &[bool], buf: &mut String) {
     digits
         .iter()
         .zip(pattern)
         .map(|(d, b)| if *b { '*' } else { *d as char })
-        .collect()
+        .for_each(|c| buf.push(c));
 }
 
 fn replace_digits_with_new_digit_according_to_pattern(