function oddEvenList(head: ListNode | null): ListNode | null {
if(!head) return null
let odd = head
let evenFirst = odd.next
let even = odd.next
let oddTurn = true
while(odd && even) {
if(oddTurn) {
if(!even.next) break
odd.next = even.next
odd = odd.next
oddTurn = false
} else {
even.next = odd.next
even = even.next
oddTurn = true
}
}
odd.next = evenFirst
return head
};