feat: add Redundant Connection (#1181)

Co-authored-by: David Leal <halfpacho@gmail.com>

leetcode/DIRECTORY.md

@@ -88,6 +88,7 @@
 | 647  | [Palindromic Substring](https://leetcode.com/problems/palindromic-substrings/)                                                  | [C](./src/647.c)  | Medium     |
 | 669  | [Trim a Binary Search Tree](https://leetcode.com/problems/trim-a-binary-search-tree/)                                           | [C](./src/669.c)  | Medium     |
 | 674  | [Longest Continuous Increasing Subsequence](https://leetcode.com/problems/longest-continuous-increasing-subsequence/)           | [C](./src/674.c)  | Easy       |
+| 684  | [Redundant Connection](https://leetcode.com/problems/redundant-connection/description/)                                         | [C](./src/684.c)  | Medium     |
 | 700  | [Search in a Binary Search Tree](https://leetcode.com/problems/search-in-a-binary-search-tree/)                                 | [C](./src/700.c)  | Easy       |
 | 701  | [Insert into a Binary Search Tree](https://leetcode.com/problems/insert-into-a-binary-search-tree/)                             | [C](./src/701.c)  | Medium     |
 | 704  | [Binary Search](https://leetcode.com/problems/binary-search/)                                                                   | [C](./src/704.c)  | Easy       |

leetcode/src/684.c

@@ -0,0 +1,49 @@
+/**
+ * Note: The returned array must be malloced, assume caller calls free().
+ */
+int find(int* sets, int index){
+    while (sets[index] != index){
+        index = sets[index];
+    }
+
+    return index;
+}
+
+void unionSet(int* sets, int i1, int i2){
+    int i1Parent = find(sets, i1);
+    int i2Parent = find(sets, i2);
+    
+    sets[i1Parent] = i2Parent;
+}
+
+// Union find
+// Runtime: O(n)
+// Space: O(n)
+int* findRedundantConnection(int** edges, int edgesSize, int* edgesColSize, int* returnSize){
+    int setsSize = edgesSize + 1;
+    int* sets = malloc(setsSize * sizeof(int));
+    for (int i = 0; i < setsSize; i++){
+        sets[i] = i;
+    }
+
+    int* result = malloc(2 * sizeof(int));
+    *returnSize = 2;
+
+    for (int i = 0; i < edgesSize; i++){
+        int* edge = edges[i];
+
+        int i0Parent = find(sets, edge[0]);
+        int i1Parent = find(sets, edge[1]);
+
+        if (i0Parent == i1Parent){
+            result[0] = edge[0];
+            result[1] = edge[1];
+            continue;
+        }
+
+        unionSet(sets, i0Parent, i1Parent);
+    }
+
+    free(sets);
+    return result;
+}