Merge branch 'jakwings-patch'

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;
 
@@ -82,7 +88,7 @@ class Solution {
 
         return l;
     }
-private:
+
     ListNode* mergeTheRest(ListNode* l, ListNode*head, ListNode* tail){
         if (l){
             if (head && tail ){
@@ -93,4 +99,28 @@ class Solution {
         }
         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;
+        while (l1 != NULL && l2 != NULL) {
+            if (l1->val < l2->val) {
+                *pTail = l1;
+                l1 = l1->next;
+            } else {
+                *pTail = l2;
+                l2 = l2->next;
+            }
+            pTail = &(*pTail)->next;
+        }
+        *pTail = (l1 != NULL ? l1 : l2);
+        return head;
+    }
+
 };