Reorder List Solution

Author: PDKhan

reorder-list/PDKhan.cpp

@@ -0,0 +1,73 @@
+// space O(n)
+class Solution {
+    public:
+        void reorderList(ListNode* head) {
+            ListNode* search = head->next;
+            ListNode* tail = head;
+            deque<ListNode*> q;
+    
+            while(search){
+                ListNode* next = search->next;
+    
+                search->next = NULL;
+                q.push_back(search);
+                search = next;
+            }
+    
+            for(int i = 0; !q.empty(); i++){
+                if(i % 2 == 0){
+                    tail->next = q.back();
+                    q.pop_back();
+                }else{
+                    tail->next = q.front();
+                    q.pop_front();
+                }
+    
+                tail = tail->next;
+            }
+        }
+    };
+
+
+// space O(1)
+class Solution {
+    public:
+        void reorderList(ListNode* head) {
+            if(head == NULL || head->next == NULL)
+                return;
+    
+            ListNode* slow = head;
+            ListNode* fast = head;
+    
+            while(fast->next && fast->next->next){
+                slow = slow->next;
+                fast = fast->next->next;
+            }
+    
+            ListNode* prev = nullptr;
+            ListNode* curr = slow->next;
+    
+            while(curr){
+                ListNode* next = curr->next;
+                curr->next = prev;
+                prev = curr;
+                curr = next;
+            }
+    
+            slow->next = nullptr;
+    
+            ListNode* first = head;
+            ListNode* second = prev;
+    
+            while(first && second){
+                ListNode* next1 = first->next;
+                ListNode* next2 = second->next;
+    
+                first->next = second;
+                second->next = next1;
+    
+                first = next1;
+                second = next2;
+            }
+        }
+    };