java: add «1346. Check If N and Its Double Exist»

URL:	https://leetcode.com/problems/check-if-n-and-its-double-exist/
Signed-off-by: Matej Focko <me@mfocko.xyz>

java/check-if-n-and-its-double-exist.java

@@ -0,0 +1,16 @@
+import java.util.HashSet;
+
+class Solution {
+  public boolean checkIfExist(int[] arr) {
+    var seen = new HashSet<Integer>();
+    for (var x : arr) {
+      if (seen.contains(2 * x) || ((x & 1) == 0 && seen.contains(x / 2))) {
+        return true;
+      }
+
+      seen.add(x);
+    }
+
+    return false;
+  }
+}