rs: add «41. First Missing Positive»

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

rs/first-missing-positive.rs

@@ -0,0 +1,21 @@
+impl Solution {
+    pub fn first_missing_positive(nums: Vec<i32>) -> i32 {
+        let n = nums.len() + 1;
+        let mut seen = vec![false; n];
+
+        for x in nums.into_iter() {
+            if x <= 0 || (n as i32) <= x {
+                continue;
+            }
+
+            seen[x as usize - 1] = true;
+        }
+
+        1 + seen
+            .into_iter()
+            .enumerate()
+            .find(|(_, val)| !val)
+            .expect("there's always a missing positive integer")
+            .0 as i32
+    }
+}