feat: add question 19

Author: yinxin630

19.删除链表的倒数第n个节点.js

@@ -0,0 +1,46 @@
+/*
+ * @lc app=leetcode.cn id=19 lang=javascript
+ *
+ * [19] 删除链表的倒数第N个节点
+ */
+
+// @lc code=start
+/**
+ * Definition for singly-linked list.
+ * function ListNode(val) {
+ *     this.val = val;
+ *     this.next = null;
+ * }
+ */
+/**
+ * @param {ListNode} head
+ * @param {number} n
+ * @return {ListNode}
+ */
+var removeNthFromEnd = function(head, n) {
+    let a = head;
+    let b = head;
+    let t = n;
+    while (t && b.next) {
+        b = b.next;
+        t--;
+    }
+    while (b.next) {
+        a = a.next;
+        b = b.next;
+    }
+
+    if (a === b) {
+        return null;
+    } else if (t === 1) {
+        return a.next;
+    } else if (t === 2) {
+        return head.next;
+    }
+
+    a.next = a.next.next;
+
+    return head;
+};
+// @lc code=end
+