update mergeTwoLists solution, time complexity changed to O(min(m.n))

Author: advancedxy

C++/chapSorting.tex

@@ -56,23 +56,20 @@ \subsubsection{分析}
 \subsubsection{代码}
 \begin{Code}
 //LeetCode, Merge Two Sorted Lists
-// 时间复杂度O(m+n)，空间复杂度O(1)
+// 时间复杂度O(min(m,n))，空间复杂度O(1)
 class Solution {
 public:
     ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) {
-        ListNode head(-1);
-        for (ListNode* p = &head; l1 != nullptr || l2 != nullptr; p = p->next) {
-            int val1 = l1 == nullptr ? INT_MAX : l1->val;
-            int val2 = l2 == nullptr ? INT_MAX : l2->val;
-            if (val1 <= val2) {
-                p->next = l1;
-                l1 = l1->next;
-            } else {
-                p->next = l2;
-                l2 = l2->next;
-            }
+        if (l1 == nullptr) return l2;
+        if (l2 == nullptr) return l1;
+        ListNode h(-1);
+        ListNode *p = &h;
+        for (; l1 != nullptr && l2 != nullptr; p = p->next) {
+            if (l1->val > l2->val) { p->next = l2; l2 = l2->next; }
+            else { p->next = l1; l1 = l1->next; }
         }
-        return head.next;
+        p->next = l1 != nullptr ? l1 : l2;
+        return h.next;
     }
 };
 \end{Code}