feat: add rust solution to lc problem: No.2309

No.2309.Greatest English Letter in Upper and Lower Case

Author: YangFong

solution/2300-2399/2309.Greatest English Letter in Upper and Lower Case/README.md

@@ -162,6 +162,29 @@ function greatestLetter(s: string): string {
 }
 ```
 
+### **Rust**
+
+```rust
+impl Solution {
+    pub fn greatest_letter(s: String) -> String {
+        let mut arr = [0; 26];
+        for &c in s.as_bytes().iter() {
+            if c >= b'a' {
+                arr[(c - b'a') as usize] |= 1;
+            } else {
+                arr[(c - b'A') as usize] |= 2;
+            }
+        }
+        for i in (0..26).rev() {
+            if arr[i] == 3 {
+                return char::from(b'A' + i as u8).to_string();
+            }
+        }
+        "".to_string()
+    }
+}
+```
+
 ### **...**
 
 ```

solution/2300-2399/2309.Greatest English Letter in Upper and Lower Case/README_EN.md

@@ -149,6 +149,29 @@ function greatestLetter(s: string): string {
 }
 ```
 
+### **Rust**
+
+```rust
+impl Solution {
+    pub fn greatest_letter(s: String) -> String {
+        let mut arr = [0; 26];
+        for &c in s.as_bytes().iter() {
+            if c >= b'a' {
+                arr[(c - b'a') as usize] |= 1;
+            } else {
+                arr[(c - b'A') as usize] |= 2;
+            }
+        }
+        for i in (0..26).rev() {
+            if arr[i] == 3 {
+                return char::from(b'A' + i as u8).to_string();
+            }
+        }
+        "".to_string()
+    }
+}
+```
+
 ### **...**
 
 ```

solution/2300-2399/2309.Greatest English Letter in Upper and Lower Case/Solution.rs

@@ -0,0 +1,18 @@
+impl Solution {
+    pub fn greatest_letter(s: String) -> String {
+        let mut arr = [0; 26];
+        for &c in s.as_bytes().iter() {
+            if c >= b'a' {
+                arr[(c - b'a') as usize] |= 1;
+            } else {
+                arr[(c - b'A') as usize] |= 2;
+            }
+        }
+        for i in (0..26).rev() {
+            if arr[i] == 3 {
+                return char::from(b'A' + i as u8).to_string();
+            }
+        }
+        "".to_string()
+    }
+}