middle-of-the-linked-list.rs

struct Solution;

// Definition for singly-linked list.
#[derive(PartialEq, Eq, Clone, Debug)]
pub struct ListNode {
    pub val: i32,
    pub next: Option<Box<ListNode>>,
}

impl ListNode {
    #[allow(unused)]
    #[inline]
    fn new(val: i32) -> Self {
        ListNode { next: None, val }
    }
}

impl Solution {
    pub fn middle_node(head: Option<Box<ListNode>>) -> Option<Box<ListNode>> {
        let (mut fast, mut slow) = (&head.clone(), head);
        loop {
            if let Some(ref node) = fast {
                if let Some(ref next) = node.next {
                    fast = &next.next;
                    slow = slow.unwrap().next;
                    continue;
                }
            }
            break;
        }
        slow
    }
}

fn main() {
    let head = Some(Box::new(ListNode {
        val: 1,
        next: Some(Box::new(ListNode {
            val: 2,
            next: Some(Box::new(ListNode {
                val: 3,
                next: Some(Box::new(ListNode {
                    val: 4,
                    next: Some(Box::new(ListNode { val: 5, next: None })),
                })),
            })),
        })),
    }));
    println!("{:?}", Solution::middle_node(head));
}

#[cfg(test)]
mod tests {
    use crate::{ListNode, Solution};

    #[test]
    fn test() {
        let head = Some(Box::new(ListNode {
            val: 1,
            next: Some(Box::new(ListNode {
                val: 2,
                next: Some(Box::new(ListNode {
                    val: 3,
                    next: Some(Box::new(ListNode {
                        val: 4,
                        next: Some(Box::new(ListNode { val: 5, next: None })),
                    })),
                })),
            })),
        }));
        assert_eq!(
            Solution::middle_node(head),
            Some(Box::new(ListNode {
                val: 3,
                next: Some(Box::new(ListNode {
                    val: 4,
                    next: Some(Box::new(ListNode { val: 5, next: None })),
                })),
            }))
        );
    }
}