add problem 19 and test function

Author: supercoderhawk

C/1-50/19-Remove-Nth-Node-From-End-of-List.c

@@ -0,0 +1,49 @@
+/*
+ * Given a linked list, remove the nth node from the end of list and return its head.
+
+ * For example,
+
+ *  Given linked list: 1->2->3->4->5, and n = 2.
+ *  After removing the second node from the end, the linked list becomes 1->2->3->5.
+ *
+ * Note:
+ * Given n will always be valid.
+ * Try to do this in one pass.
+ * Created by supercoderx on 2017/8/9.
+ */
+#include 
+#include 
+#include "../main.h"
+
+struct ListNode *removeNthFromEnd(struct ListNode *head, int n) {
+    if (head == NULL) {
+        return head;
+    }
+    struct ListNode *p = head, *q;
+    int len = 0, id, index = 0;
+    while (p != NULL) {
+        len++;
+        p = p->next;
+    }
+    id = len - n;
+    p = head;
+    if (id == 0) {
+        head = head->next;
+        free(p);
+    } else {
+        while (index != id - 1) {
+            p = p->next;
+            index++;
+        }
+        q = p->next;
+        p->next = id == len - 1 ? NULL : q->next;
+        free(q);
+    }
+
+    return head;
+
+}
+
+void testRemoveNthFromEnd() {
+
+}