Solution.cs

namespace LeetCodeNet.G0001_0100.S0086_partition_list {

// #Medium #Two_Pointers #Linked_List #Top_Interview_150_Linked_List
// #2025_07_05_Time_0_ms_(100.00%)_Space_42.69_MB_(64.54%)

using LeetCodeNet.Com_github_leetcode;

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     public int val;
 *     public ListNode next;
 *     public ListNode(int val=0, ListNode next=null) {
 *         this.val = val;
 *         this.next = next;
 *     }
 * }
 */
public class Solution {
    public ListNode Partition(ListNode head, int x) {
        ListNode beforeHead = new ListNode(0);
        ListNode afterHead = new ListNode(0);
        ListNode before = beforeHead;
        ListNode after = afterHead;
        while (head != null) {
            if (head.val < x) {
                before.next = head;
                before = before.next;
            } else {
                after.next = head;
                after = after.next;
            }
            head = head.next;
        }
        after.next = null;
        before.next = afterHead.next;
        return beforeHead.next;
    }
}
}