Update README.md

Author: yanglbme

solution/1300-1399/1399.Count Largest Group/README.md

@@ -197,6 +197,36 @@ function countLargestGroup(n: number): number {
 }
 ```
 
+#### Rust
+
+```rust
+impl Solution {
+    pub fn count_largest_group(n: i32) -> i32 {
+        let mut cnt = vec![0; 40];
+        let mut ans = 0;
+        let mut mx = 0;
+
+        for i in 1..=n {
+            let mut s = 0;
+            let mut x = i;
+            while x > 0 {
+                s += x % 10;
+                x /= 10;
+            }
+            cnt[s as usize] += 1;
+            if mx < cnt[s as usize] {
+                mx = cnt[s as usize];
+                ans = 1;
+            } else if mx == cnt[s as usize] {
+                ans += 1;
+            }
+        }
+
+        ans
+    }
+}
+```
+
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