Insert at Head - Rust DSA

Insert a new first node by pointing it at the old head and then moving the head pointer.

Algorithm

The replay labels nodes by value, such as node(20), and never exposes object identity or memory addresses. This Rust DSA implementation uses the same small chain as the rest of the DSA track.

Basic Implementation

basic.rs

struct Node {
    value: i32,
    next: Option<Box<Node>>,
}

fn node(value: i32, next: Option<Box<Node>>) -> Option<Box<Node>> {
    Some(Box::new(Node { value, next }))
}

fn render(head: &Option<Box<Node>>) -> String {
    let mut parts: Vec<String> = Vec::new();
    let mut cursor = head.as_ref();
    while let Some(node) = cursor {
        parts.push(node.value.to_string());
        cursor = node.next.as_ref();
    }
    parts.join(" -> ") + " -> null"
}

fn delete_value(head: Option<Box<Node>>, target: i32) -> Option<Box<Node>> {
    match head {
        Some(mut n) => {
            if n.value == target {
                n.next
            } else {
                n.next = delete_value(n.next, target);
                Some(n)
            }
        }
        None => None,
    }
}

fn main() {
    let mut head = node(20, node(30, None));
    let mut new_head = Box::new(Node { value: 10, next: None });
    new_head.next = head;
    head = Some(new_head);
    println!("{}", render(&head));
}

Complexity

Time: O(1)
Space: O(1)

Implementation notes

Keep the explicit node and pointer/reference operations; array shortcuts hide the linked-list state this lesson is meant to replay.
The final output prints the chain in a deterministic a -> b -> null form for cross-language comparison.

old head The previous first node becomes the second node.

constant-time insert Only the new node and head pointer change.