Head insertion into a singly linked list is the constant-time case of list insertion: a newly allocated node takes the current head as its successor, after which the head reference is reassigned to the new node — no traversal is required because the insertion point is the reference itself. The empty-list case collapses into the general case because a null head assigns correctly as the new node's successor, so the two apparent scenarios reduce to a single unconditional rule. The concept belongs to data structures as taught through the C/C++ memory model, where nodes are heap allocations reachable only via pointers and the head pointer's scope determines whether an operation that changes the first node can propagate that change to its caller.
Head insertion into a singly linked list is the constant-time case of list insertion: a newly allocated node takes the current head as its successor, after which the head reference is reassigned to t…