LeetCode – Reverse Linked List II (Java)

Reverse a linked list from position m to n. Do it in-place and in one-pass.

For example: given 1->2->3->4->5->NULL, m = 2 and n = 4, return 1->4->3->2->5->NULL.

Analysis

Java Solution

public ListNode reverseBetween(ListNode head, int m, int n) {
    if(m==n) return head;
 
    ListNode prev = null;//track (m-1)th node
    ListNode first = new ListNode(0);//first's next points to mth
    ListNode second = new ListNode(0);//second's next points to (n+1)th
 
    int i=0;
    ListNode p = head;
    while(p!=null){
        i++;
        if(i==m-1){
            prev = p;
        }
 
        if(i==m){
            first.next = p;
        }
 
        if(i==n){
            second.next = p.next;
            p.next = null;
        }
 
        p= p.next;
    }
    if(first.next == null)
        return head;
 
    // reverse list [m, n]    
    ListNode p1 = first.next;
    ListNode p2 = p1.next;
    p1.next = second.next;
 
    while(p1!=null && p2!=null){
        ListNode t = p2.next;
        p2.next = p1;
        p1 = p2;
        p2 = t;
    }
 
    //connect to previous part
    if(prev!=null)
        prev.next = p1;
    else
        return p1;
 
    return head;
}
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  • Venk

    Simple and best solution found in LeetCode discuss


    public static ListNode reverseBetween(ListNode head, int m, int n) {

    if(head == null) return null;

    ListNode dummy = new ListNode(0); // create a dummy node to mark the head of this list
    dummy.next = head;

    ListNode pre = dummy; // make a pointer pre as a marker for the node before reversing
    for(int i = 0; i pre = 1, start = 2, then = 3
    // dummy-> 1 -> 2 -> 3 -> 4 -> 5

    for(int i=0; i1 - 3 - 2 - 4 - 5; pre = 1, start = 2, then = 4
    // second reversing: dummy->1 - 4 - 3 - 2 - 5; pre = 1, start = 2, then = 5 (finish)

    return dummy.next;

    }

  • Sruthi

    public class ReverseLinkedListMtoN2 {

    public ListNode reverseBetween(ListNode head, int m, int n) {

    if (m != n) {

    ListNode curr = head;

    int i = 1;

    ListNode prevHead = null;

    ListNode tail = null;

    ListNode prev = null;

    ListNode next = null;

    while (i < m) {

    prevHead = curr;

    curr = curr.next;

    i++;

    }

    tail = curr;

    next = curr.next;

    while (i < n) {

    prev = curr;

    curr = next;

    next = curr.next;

    curr.next = prev;

    i++;

    }

    if (m == 1)

    head = curr;

    else

    prevHead.next = curr;

    tail.next = next;

    }

    return head;

    }

  • Shivendra

    Why to create new ListNode???
    When the same stuff can be done just by rearranging the references?

    /**
    * Definition for singly-linked list.
    * public class ListNode {
    * int val;
    * ListNode next;
    * ListNode(int x) { val = x; }
    * }
    */
    public class Solution {
    public ListNode reverseBetween(ListNode head, int m, int n) {

    if(m==n)
    return head;

    ListNode mprev = null;
    ListNode nafter = null;
    ListNode mprevprev = null;
    ListNode temp = head;

    int i=1;
    while(temp!=null && i<=n){

    if(i==m-1){
    mprevprev = temp;
    }
    if(i==m){
    mprev = temp;
    }
    if(i==n){
    nafter = temp.next;
    temp.next = null;
    }
    ++i;
    temp = temp.next;

    }

    if(mprev.next == null)
    return head;

    // Reverse the link list from m to n

    ListNode x = mprev;
    ListNode y = x.next;
    x.next = nafter;

    while(x!=null && y!=null){
    ListNode t = y.next;
    y.next = x;
    x=y;
    y=t;
    }

    if(mprevprev!=null)
    mprevprev.next =x;
    else
    return x;

    return head;

    }
    }