add a solution for 684

Author: fishercoder1534

Diff for: paginated_contents/algorithms/1st_thousand/README.md

@@ -186,7 +186,7 @@
 | 687 | [Longest Univalue Path](https://leetcode.com/problems/longest-univalue-path/)                                                                                                                 | [Solution](https://github.com/fishercoder1534/Leetcode/blob/master/src/main/java/com/fishercoder/solutions/firstthousand/_687.java)                                                                  |                                                                               | Easy                             | DFS                                                                  
 | 686 | [Repeated String Match](https://leetcode.com/problems/repeated-string-match/)                                                                                                                 | [Solution](https://github.com/fishercoder1534/Leetcode/blob/master/src/main/java/com/fishercoder/solutions/firstthousand/_686.java)                                                                  |                                                                               | Easy                             |
 | 685 | [Redundant Connection II](https://leetcode.com/problems/redundant-connection-ii/)                                                                                                             | [Solution](https://github.com/fishercoder1534/Leetcode/blob/master/src/main/java/com/fishercoder/solutions/firstthousand/_685.java)                                                                  |                                                                               | Hard                             | Union Find                                                           
-| 684 | [Redundant Connection](https://leetcode.com/problems/redundant-connection/)                                                                                                                   | [Solution](https://github.com/fishercoder1534/Leetcode/blob/master/src/main/java/com/fishercoder/solutions/firstthousand/_684.java)                                                                  |                                                                               | Medium                           | Union Find                                                           
+| 684 | [Redundant Connection](https://leetcode.com/problems/redundant-connection/)                                                                                                                   | [Solution](https://github.com/fishercoder1534/Leetcode/blob/master/src/main/java/com/fishercoder/solutions/firstthousand/_684.java)                                                                  |                                                                               | Medium                           | Union Find, DFS                                                           
 | 683 | [K Empty Slots](https://leetcode.com/problems/k-empty-slots/)                                                                                                                                 | [Solution](https://github.com/fishercoder1534/Leetcode/blob/master/src/main/java/com/fishercoder/solutions/firstthousand/_683.java)                                                                  |                                                                               | Hard                             |
 | 682 | [Baseball Game](https://leetcode.com/problems/baseball-game/)                                                                                                                                 | [Solution](https://github.com/fishercoder1534/Leetcode/blob/master/src/main/java/com/fishercoder/solutions/firstthousand/_682.java)                                                                  |                                                                               | Easy                             |
 | 681 | [Next Closest Time](https://leetcode.com/problems/parents-closest-time/)                                                                                                                      | [Solution](https://github.com/fishercoder1534/Leetcode/blob/master/src/main/java/com/fishercoder/solutions/firstthousand/_681.java)                                                                  |                                                                               | Medium                           |

Diff for: src/main/java/com/fishercoder/solutions/firstthousand/_684.java

@@ -1,6 +1,8 @@
 package com.fishercoder.solutions.firstthousand;
 
+import java.util.ArrayList;
 import java.util.HashSet;
+import java.util.List;
 import java.util.Set;
 
 public class _684 {
@@ -83,4 +85,44 @@ public int[] findRedundantConnection(int[][] edges) {
             return result;
         }
     }
+
+    public static class Solution2 {
+        /**
+         * DFS, credit: https://leetcode.com/problems/redundant-connection/editorial/
+         * 1. we build the graph one edge at a time, each time, we add both edge[0] to the neighbors of edge[1] and vice versa since this is an un-directed graph;
+         * 2. as soon as we encounter an edge that can connect to each other, it must be the redundant one.
+         */
+        private static final int MAX_VERTICES = 1000;
+
+        public int[] findRedundantConnection(int[][] edges) {
+            List<Integer>[] graph = new ArrayList[MAX_VERTICES + 1];
+            for (int i = 0; i < graph.length; i++) {
+                graph[i] = new ArrayList<>();
+            }
+            Set<Integer> visited = new HashSet<>();
+            for (int[] edge : edges) {
+                visited.clear();
+                if (!graph[edge[0]].isEmpty() && !graph[edge[1]].isEmpty() && canConnect(edge[0], edge[1], graph, visited)) {
+                    return edge;
+                }
+                graph[edge[0]].add(edge[1]);
+                graph[edge[1]].add(edge[0]);
+            }
+            return null;
+        }
+
+        private boolean canConnect(int source, int target, List<Integer>[] graph, Set<Integer> visited) {
+            if (visited.add(source)) {
+                if (source == target) {
+                    return true;
+                }
+                for (int v : graph[target]) {
+                    if (canConnect(source, v, graph, visited)) {
+                        return true;
+                    }
+                }
+            }
+            return false;
+        }
+    }
 }

Diff for: src/test/java/com/fishercoder/firstthousand/_684Test.java

@@ -1,18 +1,18 @@
 package com.fishercoder.firstthousand;
 
 import com.fishercoder.solutions.firstthousand._684;
-import org.junit.BeforeClass;
-import org.junit.Test;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
 
-import static org.junit.Assert.assertArrayEquals;
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
 
 public class _684Test {
     private static _684.Solution1 solution1;
     private static int[][] edges;
     private static int[] expected;
 
-    @BeforeClass
-    public static void setup() {
+    @BeforeEach
+    public void setup() {
         solution1 = new _684.Solution1();
     }