Create Reverse Nodes in Even Length Groups.java

Author: varunu28

Medium/Reverse Nodes in Even Length Groups.java

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+/**
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     int val;
+ *     ListNode next;
+ *     ListNode() {}
+ *     ListNode(int val) { this.val = val; }
+ *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
+ * }
+ */
+class Solution {
+  public ListNode reverseEvenLengthGroups(ListNode head) {
+    if (head == null) {
+      return head;
+    }
+    int numOfNodes = 1;
+    ListNode prev = null;
+    ListNode curr = head;
+    int totalNumOfNodes = 0;
+    while (curr != null) {
+      totalNumOfNodes++;
+      curr = curr.next;
+    }
+    curr = head;
+    while (curr != null) {
+      numOfNodes = Math.min(numOfNodes, totalNumOfNodes);
+      totalNumOfNodes -= numOfNodes;
+      if (numOfNodes % 2 == 0) {
+        ListNode[] res = reverseList(curr, numOfNodes);
+        prev.next = res[0];
+        prev = curr;
+        curr = res[1];
+      } else {
+        for (int i = 0; i < numOfNodes && curr != null; i++) {
+          prev = curr;
+          curr = curr.next;
+        }
+      }
+      numOfNodes++;
+    }
+    return head;
+  }
+  
+    private ListNode[] reverseList(ListNode node, int n) {
+      ListNode prev = null;
+      ListNode curr = node;
+      ListNode post = null;
+      while (n-- > 0) {
+        post = curr.next;
+        curr.next = prev;
+        prev = curr;
+        curr = post;
+      }
+      node.next = curr;
+      return new ListNode[]{prev, post};
+  }
+}