92. Reverse Linked List II

Author: SjxSubham

92. Reverse Linked List II

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+//T.c -O(n)
+//s.C -O(1)
+
+// function used to reverse a linked list
+    ListNode* reverse(ListNode* head)
+{
+    ListNode* prev = NULL;
+    ListNode* curr = head;
+    while (curr) {
+        ListNode* next = curr->next;
+        curr->next = prev;
+        prev = curr;
+        curr = next;
+    }
+    return prev;
+}
+
+ ListNode* reverseBetween(ListNode* head, int left, int right) {
+        // function used to reverse a linked list from position m to n
+    if (left == right)
+        return head;
+  
+    // revs and revend is start and end respectively of the
+    // portion of the linked list which need to be reversed.
+    // revs_prev is previous of starting position and
+    // revend_next is next of end of list to be reversed.
+    ListNode*revs = NULL, *revs_prev = NULL;
+    ListNode*revend = NULL, *revend_next = NULL;
+  
+    // Find values of above pointers.
+    int i = 1;
+    ListNode* curr = head;
+    while (curr && i <= right) {
+        if (i < left)
+            revs_prev = curr;
+        if (i == left)
+            revs = curr;
+        if (i == right) {
+            revend = curr;
+            revend_next = curr->next;
+        }
+        curr = curr->next;
+        i++;
+    }
+    revend->next = NULL;
+    // Reverse linked list starting with revs.
+    revend = reverse(revs);
+    // If starting position was not head
+    if (revs_prev)
+        revs_prev->next = revend;
+    // If starting position was head
+    else
+        head = revend;
+    revs->next = revend_next;
+    return head;
+}
+        
+    
+};