add iteration solution for leetcode 21

Author: Zhanganjin0520

algorithms/mergeTwoSortedLists/Solution.java

@@ -1,4 +1,6 @@
 public class Solution {
+
+    //迭代，空间复杂度O(m+n)
     public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
         if (null == l1) {
             return l2;
@@ -12,4 +14,33 @@ public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
             return l2;
         }
     }
+
+    //递归，空间复杂度O(1)
+    public ListNode iterationTwoLists(ListNode l1, ListNode l2) {
+        //链表头部节点
+        ListNode preHeadNode = new ListNode(-1);
+        //返回排序链表的指针
+        ListNode prev = preHeadNode;
+        while (null != l1 && null != l2) {
+            if (l1.val <= l2.val) {
+                prev.next = l1;
+                l1 = l1.next;
+            } else {
+                prev.next = l2;
+                l2 = l2.next;
+            }
+            prev = prev.next;
+        }
+        prev.next = null == l1 ? l2 : l1;
+        return preHeadNode.next;
+    }
+
+    static class ListNode {
+        public int val;
+        public ListNode next;
+
+        public ListNode(int val) {
+            this.val = val;
+        }
+    }
 }