reverse Linked list II

rev2.php

/**
 * Definition for a singly-linked list.
 * class ListNode {
 *     public $val = 0;
 *     public $next = null;
 *     function __construct($val = 0, $next = null) {
 *         $this->val = $val;
 *         $this->next = $next;
 *     }
 * }
 */
class Solution {

    /**
     * @param ListNode $head
     * @param Integer $m
     * @param Integer $n
     * @return ListNode
     */
    function reverseBetween($head, $m, $n) {
        if ($head == null) {
            return null;
        }
        
        $prev = null;
        $cur = $head;
        $count = 1;
            
        // Move pointers forward until at start of range
        while ($count < $m) {
            $prev = $cur;
            $cur = $cur->next;
            $count += 1;
        }
        
        // For fixing the list references later
        $tail = $cur;
        $con = $prev;
        
        // Step through reversing them one at a time
        while ($count <= $n) {
            $third = $cur->next;
            $cur->next = $prev;
            $prev = $cur;
            $cur = $third;
            
            $count += 1;
        }
        
        // Fix the beginning reference.
        if ($con) { 
            // If $m != 1, ie, reversed from the middle
            $con->next = $prev;
        } else {
            // If $m == 1, ie, reversed from the start
            $head = $prev;
        }
        $tail->next = $cur;
        return $head;
    }
}