add 021

Author: andavid

README.md

@@ -48,6 +48,8 @@
 | 237  | [Delete Node in a Linked List][237]         |
 | 19   | [Remove Nth Node From End of List][019]     |
 | 206  | [Reverse Linked List][206]                  |
+| 21   | [Merge Two Sorted Lists][021]               |
+
 
 
 **其他**
@@ -86,3 +88,4 @@
 [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
 [206]: https://github.com/andavid/leetcode-java/blob/master/note/206/README.md
+[021]: https://github.com/andavid/leetcode-java/blob/master/note/021/README.md

note/021/README.md

@@ -0,0 +1,94 @@
+# [Merge Two Sorted Lists][title]
+
+## Description
+
+Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.
+
+**Example:**
+
+Input: 1->2->4, 1->3->4
+Output: 1->1->2->3->4->4
+
+## 思路一
+
+遍历两个链表，比较两个链表节点的大小，修改当前节点使其指向较小的那个节点。
+
+## [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 mergeTwoLists(ListNode l1, ListNode l2) {
+    if (l1 == null) return l2;
+    if (l2 == null) return l1;
+
+    ListNode dummy = new ListNode(-1);
+    ListNode curr = dummy;
+
+    while (l1 != null && l2 != null) {
+      if (l1.val <= l2.val) {
+        curr.next = l1;
+        l1 = l1.next;
+      } else {
+        curr.next = l2;
+        l2 = l2.next;
+      }
+      curr = curr.next;
+    }
+
+    if (l1 != null) {
+      curr.next = l1;
+    }
+
+    if (l2 != null) {
+      curr.next = l2;
+    }
+
+    return dummy.next;
+  }
+}
+```
+
+## 思路二
+
+递归。
+
+比较两个链表头节点的大小，指向较小的节点，并且该节点的 next 节点可以通过递归的方法得到。
+
+## [完整代码][src2]
+
+```java
+/**
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     int val;
+ *     ListNode next;
+ *     ListNode(int x) { val = x; }
+ * }
+ */
+class Solution {
+  public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
+    if (l1 == null) return l2;
+    if (l2 == null) return l1;
+
+    if (l1.val < l2.val) {
+      l1.next = mergeTwoLists(l1.next, l2);
+      return l1;
+    } else {
+      l2.next = mergeTwoLists(l2.next, l1);
+      return l2;
+    }
+  }
+}
+```
+
+[title]: https://leetcode.com/problems/merge-two-sorted-lists
+[src]: https://github.com/andavid/leetcode-java/blob/master/src/com/andavid/leetcode/_021/Solution.java
+[src2]: https://github.com/andavid/leetcode-java/blob/master/src/com/andavid/leetcode/_021/Solution2.java

src/com/andavid/leetcode/_021/Solution.java

@@ -0,0 +1,62 @@
+class Solution {
+
+  public static ListNode mergeTwoLists(ListNode l1, ListNode l2) {
+    if (l1 == null) return l2;
+    if (l2 == null) return l1;
+
+    ListNode dummy = new ListNode(-1);
+    ListNode curr = dummy;
+
+    while (l1 != null && l2 != null) {
+      if (l1.val <= l2.val) {
+        curr.next = l1;
+        l1 = l1.next;
+      } else {
+        curr.next = l2;
+        l2 = l2.next;
+      }
+      curr = curr.next;
+    }
+
+    if (l1 != null) {
+      curr.next = l1;
+    }
+
+    if (l2 != null) {
+      curr.next = l2;
+    }
+
+    return dummy.next;
+  }
+
+  public static class ListNode {
+    int val;
+    ListNode next;
+    ListNode(int x) {
+      val = x;
+    }
+  }
+
+  public static void main(String[] args) {
+    ListNode l11 = new ListNode(1);
+    ListNode l12 = new ListNode(2);
+    ListNode l13 = new ListNode(4);
+    ListNode l21 = new ListNode(1);
+    ListNode l22 = new ListNode(3);
+    ListNode l23 = new ListNode(4);
+    l11.next = l12;
+    l12.next = l13;
+    l13.next = null;
+    l21.next = l22;
+    l22.next = l23;
+    l23.next = null;
+
+    ListNode node = mergeTwoLists(l11, l21);
+    while(node.next != null) {
+      System.out.print(node.val + ",");
+      node = node.next;
+    }
+    System.out.print(node.val);
+  }
+
+}

src/com/andavid/leetcode/_021/Solution2.java

@@ -0,0 +1,46 @@
+class Solution2 {
+
+  public static ListNode mergeTwoLists(ListNode l1, ListNode l2) {
+    if (l1 == null) return l2;
+    if (l2 == null) return l1;
+
+    if (l1.val < l2.val) {
+      l1.next = mergeTwoLists(l1.next, l2);
+      return l1;
+    } else {
+      l2.next = mergeTwoLists(l2.next, l1);
+      return l2;
+    }
+  }
+
+  public static class ListNode {
+    int val;
+    ListNode next;
+    ListNode(int x) {
+      val = x;
+    }
+  }
+
+  public static void main(String[] args) {
+    ListNode l11 = new ListNode(1);
+    ListNode l12 = new ListNode(2);
+    ListNode l13 = new ListNode(4);
+    ListNode l21 = new ListNode(1);
+    ListNode l22 = new ListNode(3);
+    ListNode l23 = new ListNode(4);
+    l11.next = l12;
+    l12.next = l13;
+    l13.next = null;
+    l21.next = l22;
+    l22.next = l23;
+    l23.next = null;
+
+    ListNode node = mergeTwoLists(l11, l21);
+    while(node.next != null) {
+      System.out.print(node.val + ",");
+      node = node.next;
+    }
+    System.out.print(node.val);
+  }
+
+}