feat: remove nth node from end of list LeetCode (#1222)

* feat: remove nth node from end of list (leetcode #19)

* fix: update the leetcode #19 solution to introduce node pointing to head

---------

Co-authored-by: David Leal <[email protected]>

Author: mindaugl

leetcode/DIRECTORY.md

@@ -21,6 +21,7 @@
 |   14 | [Longest Common Prefix](https://leetcode.com/problems/longest-common-prefix)                                                                           | [C](./src/14.c)   | Easy       |
 |   16 | [3Sum Closest](https://leetcode.com/problems/3sum-closest)                                                                                             | [C](./src/16.c)   | Medium     |
 |   17 | [Letter Combinations of a Phone Number](https://leetcode.com/problems/letter-combinations-of-a-phone-number)                                           | [C](./src/17.c)   | Medium     |
+|   19 | [Remove Nth Node From End of List](https://leetcode.com/problems/remove-nth-node-from-end-of-list)                                                     | [C](./src/19.c)   | Medium     |
 |   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     |

leetcode/src/19.c

@@ -0,0 +1,27 @@
+/**
+ * Definition for singly-linked list.
+ * struct ListNode {
+ *     int val;
+ *     struct ListNode *next;
+ * };
+ */
+
+struct ListNode *removeNthFromEnd(struct ListNode *head, int n) {
+    struct ListNode entry, *p_free, *p = head;
+    int i, sz = 0;
+    entry.next = head;
+    while (p != NULL) {
+        p = p->next;
+        sz++;
+    }
+    for (i = 0, p = &entry; i < sz - n; i++, p = p -> next)
+    ;
+    p_free = p->next;
+    if (n != 1) {
+        p->next = p->next->next;
+    } else {
+        p->next = NULL;
+    }
+    free(p_free);
+    return entry.next;
+}