leetcode 76: Reverse Linked List II

Reverse Linked List IIJun 27 '12

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.

Note:
Given m, n satisfy the following condition:

 

 

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode reverseBetween(ListNode head, int m, int n) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if( head==null | m==n) return head;
        
        ListNode start = new ListNode(0);
        start.next = head;
        head = start;
        n=n-m+1; 
        
        while(--m>0 && start!=null) start = start.next;
        if(start==null) return head.next;
        
        ListNode cur = start.next;
        ListNode pre = null;
        ListNode nxt = null;
        
        while(cur!=null && n-->0) {
            nxt = cur.next;
            cur.next = pre;
            pre = cur;
            cur = nxt;
        }
        start.next.next = cur;
        ListNode t = start.next;
        start.next = pre;
        start = t;
        
        return head.next;
    }
}