rs: add «2441. Largest Positive Integer That Exists With Its Negative»

Signed-off-by: Matej Focko <me@mfocko.xyz>

rs/largest-positive-integer-that-exists-with-its-negative.rs

@@ -0,0 +1,19 @@
+use std::cmp;
+use std::collections::HashSet;
+
+impl Solution {
+    pub fn find_max_k(nums: Vec<i32>) -> i32 {
+        let mut seen = HashSet::<i32>::new();
+
+        let mut max_k = -1;
+        for x in &nums {
+            if seen.contains(&-x) {
+                max_k = cmp::max(max_k, x.abs());
+            }
+
+            seen.insert(*x);
+        }
+
+        max_k
+    }
+}