From 0d2be7eab976c2b084ae74fafd20a9719b1c3595 Mon Sep 17 00:00:00 2001
From: Manuel Pineda <manuelpineda@google.com>
Date: Mon, 4 Jan 2021 20:45:31 +0100
Subject: [PATCH] Create 04.cc

---
 solved/LeetCode/Challenges/2020/January/04.cc | 50 +++++++++++++++++++
 1 file changed, 50 insertions(+)
 create mode 100644 solved/LeetCode/Challenges/2020/January/04.cc

diff --git a/solved/LeetCode/Challenges/2020/January/04.cc b/solved/LeetCode/Challenges/2020/January/04.cc
new file mode 100644
index 00000000..488052cb
--- /dev/null
+++ b/solved/LeetCode/Challenges/2020/January/04.cc
@@ -0,0 +1,50 @@
+class Solution {
+public:
+    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
+        ListNode* tail = (val(l1) < val(l2)) ? l1 : l2;
+        ListNode* head = tail;
+        if (tail == l1) {
+            l1 = advance(l1);
+        } else {
+            l2 = advance(l2);
+        }
+        
+        while ((l1 != nullptr) && (l2 != nullptr)) {
+            ListNode* next = (val(l1) < val(l2)) ? l1 : l2;
+            if (next == l1) {
+                l1 = advance(l1);
+            } else {
+                l2 = advance(l2);
+            }
+            tail->next = next;
+            tail = advance(tail);
+        }
+        
+        while (l1 != nullptr) {
+            tail->next = l1;
+            tail = advance(tail);
+            l1 = advance(l1);
+        }
+        
+        while (l2 != nullptr) {
+            tail->next = l2;
+            tail = advance(tail);
+            l2 = advance(l2);
+        }
+        return head;
+    }
+private:
+    ListNode* advance(ListNode* l) {
+        if (l == nullptr) {
+            return l;
+        }
+        return l->next;
+    }
+    
+    int val(ListNode* l) {
+        if (l == nullptr) {
+            return 1000;
+        }
+        return l->val;
+    }
+};