0876-middle-of-the-linked-list.kt

/**
 * Example:
 * var li = ListNode(5)
 * var v = li.`val`
 * Definition for singly-linked list.
 * class ListNode(var `val`: Int) {
 *     var next: ListNode? = null
 * }
 */
class Solution {
    // use two pointers, fast one that is 2x fast as slow, which will reach end when slow reaches mid
    fun middleNode(head: ListNode?): ListNode? {
        if(head == null) return null
        var slow = head; var fast = head
        while(fast != null && fast?.next != null){
            slow = slow?.next
            fast = fast?.next?.next
        }
        return slow
    }
}