rs: add «442. Find All Duplicates in an Array»

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

rs/find-all-duplicates-in-an-array.rs

@@ -0,0 +1,17 @@
+impl Solution {
+    pub fn find_duplicates(mut nums: Vec<i32>) -> Vec<i32> {
+        let mut duplicates = vec![];
+
+        for i in 0..nums.len() {
+            let index = nums[i].abs() - 1;
+
+            if nums[index as usize] < 0 {
+                duplicates.push(index + 1);
+            } else {
+                nums[index as usize] *= -1;
+            }
+        }
+
+        duplicates
+    }
+}