Problem

Given the head of a singly linked list, delete the first node and return the new head. Return null if the list is empty or becomes empty.

Input / Output

• Input: head of a singly linked list.
• Output: head after deleting the first node.

Examples

Example 1 Input: 1 -> 2 -> 3 Output: 2 -> 3

Example 2 Input: 5 Output: empty

Example 3 Input: empty Output: empty

Constraints

• 0 <= number of nodes <= 10^5

Approach

1. If head is null, return null.
2. newHead = head.next.
3. Detach the old head (head.next = null) so the memory can be reclaimed, and return newHead.

Deleting the head is O(1) because a linked list has no need to shift elements — contrast with deleting index 0 of an array, which is O(n). This is the property that makes linked lists suitable for queues and LRU caches.

In languages with manual memory management (C/C++), free/delete the old node after re-pointing.

Complexity

• Time: O(1)
• Space: O(1)

Reference solution (Python)

class ListNode:
    def __init__(self, val, nxt=None):
        self.val, self.next = val, nxt

def delete_head(head):
    if head is None:
        return None
    new_head = head.next
    head.next = None
    return new_head

Test cases

Input	Expected
1->2->3	2->3
5	None
empty	None
1->1	1