drop the itertools again

src/day06.rs

@@ -1,4 +1,4 @@
-use itertools::Itertools;
+use std::collections::BTreeSet;
 
 pub fn process_part_1(input: &str) -> usize {
     find_unique_window_start(input, 4)
@@ -14,7 +14,7 @@ pub fn find_unique_window_start(input: &str, window_size: usize) -> usize {
             .chars()
             .collect::<Vec<_>>()
             .windows(window_size)
-            .take_while(|x| x.iter().unique().count() != x.len())
+            .take_while(|x| x.iter().collect::<BTreeSet<_>>().len() != x.len())
             .count()
 }