Remove Nth Node From End of List: the idea to remember.

The core insight

Start from a dummy head, move fast n nodes ahead, and move both pointers until fast reaches the last node.

Why it works

The fixed gap makes slow stop immediately before the node to delete. A dummy predecessor handles removal of the head uniformly.

The mistake to avoid

Return dummy.next because the original head might be removed. The loop condition depends on the exact initial gap.

Commented Java solution

Use java.util.* imports and a surrounding solution class. Tree snippets assume the standard TreeNode definition; linked lists use ListNode and Clone Graph uses a Node with val and neighbors; Copy Random List uses Node with val, next and random. Read the input assumptions above.

ListNode removeNthFromEnd(ListNode head, int n) {
    ListNode dummy = new ListNode(0); dummy.next = head;
    ListNode slow = dummy, fast = dummy;
    for (int i = 0; i < n; i++) fast = fast.next;
    while (fast.next != null) { slow = slow.next; fast = fast.next; }
    slow.next = slow.next.next;
    return dummy.next;
}

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