add some comments

Author: haoel

src/mergeTwoSortedList/mergeTwoSortedList.cpp

@@ -23,12 +23,17 @@ class Solution {
         srand(time(NULL));
     }
     ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) {
-        if (random()%2){
-            return mergeTwoLists01(l1, l2);
+        switch (random()%2){
+            case 0:
+                return mergeTwoLists01(l1, l2);
+            case 1:
+                return mergeTwoLists02(l1, l2);
+            default:
+                return mergeTwoLists03(l1, l2);
         }
-        return mergeTwoLists02(l1, l2);
     }
 
+    /* merge the 2nd list into 1st list*/
     ListNode *mergeTwoLists01(ListNode* head1, ListNode* head2){
         ListNode *p1 = head1,  *p2=head2;
         static ListNode dummy(0);
@@ -55,6 +60,7 @@ class Solution {
     }
 
 
+    /* merge two lists to the new list */
     ListNode *mergeTwoLists02(ListNode *l1, ListNode *l2) {
         ListNode *l=NULL, *p=NULL;
 
@@ -83,6 +89,23 @@ class Solution {
         return l;
     }
 
+    ListNode* mergeTheRest(ListNode* l, ListNode*head, ListNode* tail){
+        if (l){
+            if (head && tail ){
+                tail->next = l;
+            }else{
+                head = l;
+            }
+        }
+        return head;
+    }
+
+    /*  
+     * You can see the 2nd slution's code is quite complicated, 
+     * because it need to check the (head==NULL) situation.
+     * We can use the "pointer to pointer" to make the code more clean
+     * however, this would be bad for performance.
+     */
     ListNode *mergeTwoLists03(ListNode *l1, ListNode *l2) {
         ListNode *head = NULL;
         ListNode **pTail = &head;
@@ -100,15 +123,4 @@ class Solution {
         return head;
     }
 
-private:
-    ListNode* mergeTheRest(ListNode* l, ListNode*head, ListNode* tail){
-        if (l){
-            if (head && tail ){
-                tail->next = l;
-            }else{
-                head = l;
-            }
-        }
-        return head;
-    }
 };