Add C Solutions for 3 problems: 141-Linked-List-Cycle.c, 206-Reverse-Linked-List.c and 21-Merge-Two-Sorted-Lists

Author: th-blitz

Diff for: c/141-Linked-List-Cycle.c

@@ -0,0 +1,35 @@
+
+/**
+ * Definition for singly-linked list.
+ * struct ListNode {
+ *     int val;
+ *     struct ListNode *next;
+ * };
+ */
+
+bool Traverse(struct ListNode* slow, struct ListNode* fast) {
+
+    if (slow == NULL || slow -> next == NULL) {
+        return false;
+    }
+
+    if (fast == NULL || fast -> next == NULL) {
+        return false;
+    }
+
+    if (slow == fast) {
+        return true;
+    }
+    return Traverse(slow -> next, fast -> next -> next);
+}
+
+
+bool hasCycle(struct ListNode *head) {
+    
+    if (head == NULL) {
+        return false;
+    }
+    
+    return Traverse(head, head -> next);
+
+}

Diff for: c/206-Reverse-Linked-List.c

@@ -0,0 +1,24 @@
+
+/**
+ * Definition for singly-linked list.
+ * struct ListNode {
+ *     int val;
+ *     struct ListNode *next;
+ * };
+ */
+
+
+struct ListNode* reverseList(struct ListNode* head){
+    
+    if (head == NULL) {
+        return NULL;
+    } else if (head -> next == NULL) {
+        return head;
+    }
+    
+    struct ListNode* lastNode = reverseList(head -> next);
+    head -> next -> next = head;
+    head -> next = NULL;
+    return lastNode;
+    
+}

Diff for: c/21-Merge-Two-Sorted-Lists.c

@@ -0,0 +1,46 @@
+
+/**
+ * Definition for singly-linked list.
+ * struct ListNode {
+ *     int val;
+ *     struct ListNode *next;
+ * };
+ */
+
+
+struct ListNode* mergeTwoLists(struct ListNode* list1, struct ListNode* list2){
+    
+    if (list1 == NULL && list2 == NULL) {
+        return NULL;
+    } else if (list1 != NULL && list2 == NULL) {
+        return list1;
+    } else if (list2 != NULL && list1 == NULL) {
+        return list2;
+    }
+
+    struct ListNode* temp_node = NULL;
+
+    if (list1 -> val < list2 -> val) {
+        temp_node = list1;
+        list1 = list1 -> next;
+    } else {
+        temp_node = list2;
+        list2 = list2 -> next;
+    }
+
+    struct ListNode* root = temp_node;
+    
+    while((list1 != NULL) || (list2 != NULL)) {
+
+        if ((list2 == NULL) || ((list1 != NULL) && (list1 -> val < list2 -> val))) {
+            temp_node -> next = list1;
+            list1 = list1 -> next;
+        } else {
+            temp_node -> next = list2;
+            list2 = list2 -> next;
+        }
+        temp_node = temp_node -> next;
+    }
+
+    return root;
+}