Partition List.java

/*
Given a linked list and a value x, partition it such that all nodes less than x come before nodes greater than or equal to x.

You should preserve the original relative order of the nodes in each of the two partitions.

For example,
Given 1->4->3->2->5->2 and x = 3,
return 1->2->2->4->3->5. 
*/

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode partition(ListNode head, int x) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        ListNode beforeStart = null;
        ListNode beforeEnd = null;
        ListNode afterStart = null;
        ListNode afterEnd = null;
        
        if (head == null) return null;
        ListNode node = head;
        while (node != null) {
            ListNode next = node.next;
            node.next = null;
            if (node.val < x) {
                if (beforeStart == null) {
                    beforeStart = node;
                    beforeEnd = node;
                } else {
                    beforeEnd.next = node;
                    beforeEnd = node;
                }
            } else {
                if (afterStart == null) {
                    afterStart = node;
                    afterEnd = node;
                } else {
                    afterEnd.next = node;
                    afterEnd = node;
                }
            }
            node = next;
        }
        if (beforeEnd == null) {
            return afterStart;
        } else {
            beforeEnd.next = afterStart;
            return beforeStart;
        }
        
    }
}