add problem 21 and test function

Author: supercoderhawk

C/1-50/21-Merge-Two-Sorted-Lists.c

@@ -0,0 +1,59 @@
+/*
+ * 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.
+ * Created by supercoderx on 2017/8/10.
+ */
+
+#include 
+#include 
+#include "../main.h"
+
+struct ListNode* mergeTwoLists(struct ListNode* l1, struct ListNode* l2) {
+    struct ListNode *p1 = l1,*p2=l2, *header=NULL,*p3=NULL,*node=NULL;
+    if(l1==NULL || l2 == NULL){
+        return l1==NULL?l2:l1;
+    }
+
+    while (p1!=NULL || p2!=NULL){
+        node = malloc(sizeof(struct ListNode));
+        node->next = NULL;
+        if(p1==NULL){
+            node->val = p2->val;
+            p3->next = node;
+            p3 = node;
+            p2=p2->next;
+        }
+        else if(p2==NULL){
+            node->val = p1->val;
+            p3->next = node;
+            p3 = node;
+            p1=p1->next;
+        } else{
+            if(p1->val<p2->val){
+                node->val = p1->val;
+                p1=p1->next;
+            }else{
+                node->val = p2->val;
+                p2=p2->next;
+            }
+            if(header == NULL){
+                header = node;
+            }else{
+                p3->next = node;
+            }
+            p3 = node;
+        }
+    }
+    return header;
+}
+
+void testMergeTwoLists(){
+    int a1[]={1,2,3},b1[]={4,5,6};
+    int a2[]={1,9,10},b2[]={2,3,4,6,7};
+    struct ListNode * la1 = createList(a1,3);
+    struct ListNode * lb1 = createList(b1,3);
+    struct ListNode * la2 = createList(a2,3);
+    struct ListNode * lb2 = createList(b2,5);
+    printList(mergeTwoLists(la1,lb1));
+    printList(mergeTwoLists(la2,lb2));
+
+}