yet another nit (#85)

Co-authored-by: Luke Videckis <lukevideckis@gmail.com>

Author: lrvideckis

src/data_structures/trie.rs

@@ -1,7 +1,7 @@
 //! # Trie
 
 const ALPHABET_SIZE: usize = 26;
-const FIRST_CHAR: char = 'A';
+const FIRST_CHAR: usize = 'A' as usize;
 
 #[derive(Default)]
 struct Node {
@@ -45,7 +45,7 @@ impl Trie {
     pub fn insert(&mut self, s: &[char]) {
         let mut v = 0;
         for &ch in s {
-            let idx = ch as usize - FIRST_CHAR as usize;
+            let idx = ch as usize - FIRST_CHAR;
             if self.t[v].next[idx].is_none() {
                 self.t[v].next[idx] = Some(self.t.len());
                 self.t.push(Node::default());
@@ -63,7 +63,7 @@ impl Trie {
     pub fn find(&self, s: &[char]) -> usize {
         let mut v = 0;
         for &ch in s {
-            let idx = ch as usize - FIRST_CHAR as usize;
+            let idx = ch as usize - FIRST_CHAR;
             if let Some(u) = self.t[v].next[idx] {
                 v = u;
             } else {