Solution for Leedcode problem 2 and 21

leetcode/README.md

@@ -6,10 +6,12 @@ LeetCode
 
 | # | Title | Solution | Difficulty |
 |---| ----- | -------- | ---------- |
-|1|[Two Sum](https://leetcode.com/problems/two-sum/) | [C](./src/1.c)|Easy
+|1|[Two Sum](https://leetcode.com/problems/two-sum/) | [C](./src/1.c)|Easy|
+|2|[Add Two Numbers](https://leetcode.com/problems/add-two-numbers/) | [C](./src/2.c)|Medium|
 |3|[Longest Substring Without Repeating Characters](https://leetcode.com/problems/longest-substring-without-repeating-characters/) | [C](./src/3.c)|Medium|
 |4|[Median of Two Sorted Arrays](https://leetcode.com/problems/median-of-two-sorted-arrays/) | [C](./src/4.c)|Hard|
 |20|[Valid Parentheses](https://leetcode.com/problems/valid-parentheses/) | [C](./src/20.c)|Easy|
+|21|[Merge Two Sorted Lists](https://leetcode.com/problems/merge-two-sorted-lists/) | [C](./src/21.c)|Easy|
 |24|[Swap Nodes in Pairs](https://leetcode.com/problems/swap-nodes-in-pairs/) | [C](./src/24.c)|Medium|
 |26|[Remove Duplicates from Sorted Array](https://leetcode.com/problems/remove-duplicates-from-sorted-array/) | [C](./src/26.c)|Easy|
 |27|[Remove Element](https://leetcode.com/problems/remove-element/) | [C](./src/27.c)|Easy|

leetcode/src/2.c

@@ -0,0 +1,51 @@
+/*
+ * Definition for singly-linked list.
+ * struct ListNode {
+ *     int val;
+ *     struct ListNode *next;
+ * };
+ */
+
+struct ListNode* addTwoNumbers(struct ListNode* l1, struct ListNode* l2) {
+    struct ListNode *head = NULL;
+    struct ListNode *walk = NULL;
+    struct ListNode *tmp = NULL;
+
+    int carry = 0;
+    int val1 = 0;
+    int val2 = 0;
+    int val = 0;
+
+    while(l1 != NULL || l2 != NULL || carry) {
+            val1 = 0;
+            val2 = 0;
+            val = 0;
+
+            if(l1) {
+                val1 = l1->val;
+                l1 = l1->next;
+            }
+
+            if(l2) {
+                val2 = l2->val;
+                l2 = l2->next;
+            }
+
+            val = carry + val1 + val2;
+            carry = val / 10;
+
+            tmp = malloc(sizeof(struct ListNode));
+            tmp->val = val % 10;
+            tmp->next = NULL;
+
+            if(!head) {
+                head = walk = tmp;
+            } else {
+                walk->next = tmp;
+                walk = walk->next;
+            }
+        }
+
+        return head;
+}
+

leetcode/src/21.c

@@ -0,0 +1,60 @@
+/*
+ * Iterative approach
+ */
+struct ListNode* mergeTwoLists(struct ListNode* l1, struct ListNode* l2) {
+    struct ListNode *list = NULL;
+    struct ListNode *tmp = NULL;
+
+    if (!l1)
+        return l2;
+    if (!l2)
+        return l1;
+
+    if (l1 && l2) {
+        if (l1->val < l2->val) {
+            list = tmp = l1;
+            l1 = l1->next;
+        } else {
+            list = tmp = l2;
+            l2 = l2->next;
+        }
+
+        while(l1 && l2) {
+            if (l1->val < l2->val) {
+                tmp->next = l1;
+                l1 = l1->next;
+            } else {
+                tmp->next = l2;
+                l2 = l2->next;
+            }
+            tmp = tmp->next;
+        }
+
+        if (l1)
+            tmp->next = l1;
+        if (l2)
+            tmp->next = l2;
+
+        return list;
+    }
+
+    return NULL;
+}
+
+/*
+ * Recursive approach
+ */
+struct ListNode* mergeTwoLists(struct ListNode* l1, struct ListNode* l2) {
+    if(!l1)
+        return l2;
+    if(!l2)
+        return l1;
+    if(l1->val < l2->val) {
+        l1->next = mergeTwoLists(l1->next, l2);
+        return l1;
+    } else {
+        l2->next = mergeTwoLists(l1, l2->next);
+        return l2;
+    }
+}
+