Create 21_Merge Two Sorted Lists

Author: CY-Gu

Diff for: 21_Merge Two Sorted Lists

@@ -0,0 +1,40 @@
+/**
+ * Definition for singly-linked list.
+ * struct ListNode {
+ *     int val;
+ *     ListNode *next;
+ *     ListNode(int x) : val(x), next(NULL) {}
+ * };
+ */
+class Solution {
+public:
+    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
+        
+        ListNode *result = new ListNode();
+        ListNode *r=  result;
+        ListNode *first = r; //first用來記錄一開始的位址，否則之後找不回來(因r會變動)
+        
+        while(l1 && l2){
+
+            if(l1->val <= l2->val){
+                r->next = new ListNode(l1->val);
+                l1 = l1->next;          
+            }
+            else {
+                r->next = new ListNode(l2->val);
+                l2 = l2->next;          
+            }
+            r = r->next;
+        }
+        if(l1)
+            r->next = l1;
+            // 若l1還有，l2沒有，則l1全部加入r
+            // 為啥可以直接加入?
+            // 因前面while已比過大小，此時剩下的l1，必定是比較大的數
+        if(l2)
+            r->next = l2;
+     
+        return first->next;
+        
+    }
+};