fix: resolve SpotBugs in Yens algorithm

Author: SamXop123

src/main/java/com/thealgorithms/graph/YensKShortestPaths.java

@@ -87,9 +87,8 @@ public static List<List<Integer>> kShortestPaths(int[][] weights, int src, int d
                     long totalCost = rootCost + spurPath.cost; // spurPath.cost covers from spurNode to dst
                     Path candidate = new Path(totalNodes, totalCost);
                     String key = candidate.key();
-                    if (!seen.contains(key)) {
+                    if (seen.add(key)) {
                         b.add(candidate);
-                        seen.add(key);
                     }
                 }
             }