Merge in between linked lists

Linked List

82.9% acceptance

Feb 25, 2026

2256

227

You are given two linked lists: list1 and list2 of sizes n and m respectively. Remove list1's nodes from the ath node to the bth node, and put list2 in their place.

Solution

Rust

Time O(n)

Space O(n)

LeetCode

solution.rs

impl Solution {
  pub fn merge_in_between(
    list1: Option<Box<ListNode>>,
    a: i32,
    b: i32,
    list2: Option<Box<ListNode>>,
  ) -> Option<Box<ListNode>> {
    // Convert to Vec, splice, convert back
    let mut v1 = vec![];
    let mut cur = list1;
    while let Some(node) = cur {
      v1.push(node.val);
      cur = node.next;
    }
    let mut v2 = vec![];
    let mut cur = list2;
    while let Some(node) = cur {
      v2.push(node.val);
      cur = node.next;
    }
    let a = a as usize;
    let b = b as usize;
    let merged: Vec<i32> = v1[..a].iter()
      .chain(v2.iter())
      .chain(v1[b + 1..].iter())
      .cloned()
      .collect();
    // Build linked list from merged
    let mut head = None;
    for &val in merged.iter().rev() {
      let mut node = Box::new(ListNode::new(val));
      node.next = head;
      head = Some(node);
    }
    head
  }

  // Helper to collect list values
  pub fn to_vec(list: Option<Box<ListNode>>) -> Vec<i32> {
    let mut v = vec![];
    let mut cur = list;
    while let Some(node) = cur {
      v.push(node.val);
      cur = node.next;
    }
    v
  }
}