143. Reorder List

Medium

You are given the head of a singly linked-list. The list can be represented as:

L0 → L1 → … → Ln - 1 → Ln

Reorder the list to be on the following form:

L0 → Ln → L1 → Ln - 1 → L2 → Ln - 2 → …

You may not modify the values in the list's nodes. Only nodes themselves may be changed.

Example 1:

Input: head = [1,2,3,4]
Output:
 [1,4,2,3]

Example 2:

Input: head = [1,2,3,4,5]
Output:
 [1,5,2,4,3]

Constraints:

• The number of nodes in the list is in the range [1, 5 * 104].

• 1 <= Node.val <= 1000

解題

/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */
func reorderList(head *ListNode)  {
    fast, slow, prev := head, head, head
    stack := make([]*ListNode, 0)

    for  fast!= nil && fast.Next != nil  {
        fast = fast.Next.Next
        prev = slow
        slow = slow.Next
    }

    
    for slow != nil {
        stack = append(stack, slow)
        slow = slow.Next
    }
    prev.Next = nil

    h := head
    for h!= nil && len(stack) > 0 {
        temp := h.Next
        h.Next = stack[len(stack) - 1]
        stack[len(stack) - 1].Next = temp
        h = temp

        if h == nil {
            h = stack[len(stack) - 1]
        }

        stack = stack[:len(stack) - 1]
    }
}