add 019

Author: andavid

README.md

@@ -46,6 +46,7 @@
 | #    | Title                                       |
 | :--: | :------------------------------------------ |
 | 237  | [Delete Node in a Linked List][237]         |
+| 19   | [Remove Nth Node From End of List][019]     |
 
 
 
@@ -83,3 +84,4 @@
 [038]: https://github.com/andavid/leetcode-java/blob/master/note/038/README.md
 [014]: https://github.com/andavid/leetcode-java/blob/master/note/014/README.md
 [237]: https://github.com/andavid/leetcode-java/blob/master/note/237/README.md
+[019]: https://github.com/andavid/leetcode-java/blob/master/note/019/README.md

note/019/README.md

@@ -0,0 +1,59 @@
+# [Remove Nth Node From End of List][title]
+
+## Description
+
+Given a linked list, remove the n<sup>th</sup> 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.
+
+## 思路
+
+如果知道链表的长度，就可以从前往后一次遍历就可以了。但是求链表长度还需要遍历一次，因此总共需要两次遍历。题目要求使用一次遍历，可以使用两个指针来实现。初始都指向 head，第一个指针前进 N 步，然后两个指针同时前进直到第一个指针到达链表末尾，第二个指针后面的那个节点就是要移除的节点。
+
+## [完整代码][src]
+
+```java
+/**
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     int val;
+ *     ListNode next;
+ *     ListNode(int x) { val = x; }
+ * }
+ */
+class Solution {
+  public ListNode removeNthFromEnd(ListNode head, int n) {
+    ListNode first = head;
+    ListNode second = head;
+
+    for (int i = 1; i <= n; i++) {
+      if (first.next == null) {
+        return head.next;
+      }
+      first = first.next;
+    }
+
+    while (first.next != null) {
+      first = first.next;
+      second = second.next;
+    }
+
+    second.next = second.next.next;
+    return head;
+  }
+}
+```
+
+[title]: https://leetcode.com/problems/remove-nth-node-from-end-of-list
+[src]: https://github.com/andavid/leetcode-java/blob/master/src/com/andavid/leetcode/_019/Solution.java

src/com/andavid/leetcode/_019/Solution.java

@@ -0,0 +1,51 @@
+class Solution {
+
+  public static ListNode removeNthFromEnd(ListNode head, int n) {
+    ListNode first = head;
+    ListNode second = head;
+
+    for (int i = 1; i <= n; i++) {
+      if (first.next == null) {
+        return head.next;
+      }
+      first = first.next;
+    }
+
+    while (first.next != null) {
+      first = first.next;
+      second = second.next;
+    }
+
+    second.next = second.next.next;
+    return head;
+  }
+
+  public static class ListNode {
+    int val;
+    ListNode next;
+    ListNode(int x) {
+      val = x;
+    }
+  }
+
+  public static void main(String[] args) {
+    ListNode node1 = new ListNode(1);
+    ListNode node2 = new ListNode(2);
+    ListNode node3 = new ListNode(3);
+    ListNode node4 = new ListNode(4);
+    ListNode node5 = new ListNode(5);
+    node1.next = node2;
+    node2.next = node3;
+    node3.next = node4;
+    node4.next = node5;
+    node5.next = null;
+
+    ListNode node = removeNthFromEnd(node1, 2);
+    while(node.next != null) {
+      System.out.print(node.val + ",");
+      node = node.next;
+    }
+    System.out.print(node.val);
+  }
+
+}