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Problem solution · Java

Reverse Nodes in Even Length Groups

Reverse Nodes in Even Length Groups: a Java solution using direct simulation. Learn the idea, check the complexity, and read the full code, with credit to walkccc LeetCode Solutions.

Technique
Direct simulation
Source
walkccc LeetCode Solutions
Length
54 lines
Start with the idea.

Try the problem first. If you get stuck, read the approach below, then write your own solution. The full code is at the bottom.

Approach

Direct simulation

For Reverse Nodes in Even Length Groups, the implementation follows the problem’s operations directly while maintaining only the state needed for the next decision.

  1. Translate each rule into one explicit state update.
  2. Maintain the invariant after every processed item.
  3. Return the accumulated state once all relevant input has been handled.

Code notes

  • 54 lines of Java from the credited upstream file 2074.java.
  • The implementation keeps its working state in language-native values and containers.
  • 3 loop blocks detected.

Complexity

Count the number and nesting of passes over the input, then include the maintained containers in the memory estimate.

Check the problem constraints before deciding whether this complexity will pass.

Source

Code and credit

This code comes from walkccc LeetCode Solutions by P.-Y. Chen (walkccc) and is used under the MIT licence.

Open problem ↗Original source ↗
Full codeReverse Nodes in Even Length Groups · JavaJava
Use this to learn the idea, then write your own version.
class Solution {  public ListNode reverseEvenLengthGroups(ListNode head) {    // prev -> (head -> ... -> tail) -> next -> ...    ListNode dummy = new ListNode(0, head);    ListNode prev = dummy;    ListNode tail = head;    ListNode next = head.next;    int groupLength = 1;     while (true) {      if (groupLength % 2 == 1) {        prev.next = head;        prev = tail;      } else {        tail.next = null;        prev.next = reverse(head);        // Prev -> (tail -> ... -> head) -> next -> ...        head.next = next;        prev = head;      }      if (next == null)        break;      head = next;      Pair<ListNode, Integer> res = getTailAndLength(head, groupLength + 1);      tail = res.getKey();      next = tail.next;      groupLength = res.getValue();    }     return dummy.next;  }   private Pair<ListNode, Integer> getTailAndLength(ListNode head, int groupLength) {    int length = 1;    ListNode tail = head;    while (length < groupLength && tail.next != null) {      tail = tail.next;      ++length;    }    return new Pair<>(tail, length);  }   ListNode reverse(ListNode head) {    ListNode prev = null;    while (head != null) {      ListNode next = head.next;      head.next = prev;      prev = head;      head = next;    }    return prev;  }}

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