leetcode new problems

leetcode/lc21/MergeLists.cpp

@@ -0,0 +1,36 @@
+/**
+ * Definition for singly-linked list.
+ * struct ListNode {
+ *     int val;
+ *     ListNode *next;
+ *     ListNode() : val(0), next(nullptr) {}
+ *     ListNode(int x) : val(x), next(nullptr) {}
+ *     ListNode(int x, ListNode *next) : val(x), next(next) {}
+ * };
+ */
+class Solution {
+public:
+    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
+        ListNode* dummy = new ListNode(0);
+        ListNode* cur = dummy;
+
+        while(l1 && l2){
+            if((l1->val) < (l2->val) ){
+                cur->next = l1;
+                l1 = l1->next;
+            }
+            else{
+                cur->next = l2;
+                l2 = l2->next;
+            }
+            cur = cur->next;
+        }
+        cur->next = l1? l1 : l2;
+        ListNode* head = dummy->next;
+        delete dummy;
+        return head;
+
+    }
+};
+// TC : O(m+n)
+// SC : O(1)

leetcode/lc21/RecursiveMerge.cpp

@@ -0,0 +1,17 @@
+class Solution{
+    public : 
+        ListNode* mergeTwoLists(ListNode* l1, ListNode* l2){
+            if(!l1 || !l2)
+                return l1? l1: l2;
+            if(l1->val > l2->val)
+                swap(l1,l2);
+            l1->next = mergeTwoLists(l1->next, l2);
+            return l1;
+
+        }
+}
+
+//At every step, pick the smaller head node, attach it to the result,and recursively merge the remaining lists
+//TC : O(m+n)
+//SC: O(m+n), recursion stack space
+