Merge In Between Linked Lists

Problem

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.

The blue edges and nodes in the following figure indicate the result:

Build the result list and return its head.

Examples

Example 1

![](https://assets.leetcode.com/uploads/2024/03/01/ll.png)

Input: list1 = [10,1,13,6,9,5], a = 3, b = 4, list2 = [1000000,1000001,1000002]
Output: [10,1,13,1000000,1000001,1000002,5]
Explanation: We remove the nodes 3 and 4 and put the entire list2 in their place. The blue edges and nodes in the above figure indicate the result.

Example 2

![](https://assets.leetcode.com/uploads/2020/11/05/merge_linked_list_ex2.png)

Input: list1 = [0,1,2,3,4,5,6], a = 2, b = 5, list2 = [1000000,1000001,1000002,1000003,1000004]
Output: [0,1,1000000,1000001,1000002,1000003,1000004,6]
Explanation: The blue edges and nodes in the above figure indicate the result.

Constraints

• 3 <= list1.length <= 10^4
• 1 <= a <= b < list1.length - 1
• 1 <= list2.length <= 10^4

Solution

Method 1 – Pointer Manipulation

Intuition

To merge list2 into list1 between nodes a and b, we need to find the node just before a and the node just after b in list1. We then connect the end of list2 to the node after b, and the node before a to the head of list2.

Approach

1. Traverse list1 to find the node before position a (prev_a).
2. Traverse to find the node at position b (after_b).
3. Traverse list2 to find its tail.
4. Connect prev_a.next to list2's head.
5. Connect list2's tail to after_b.next.
6. Return the head of list1.

Code

C++

struct ListNode {
    int val;
    ListNode *next;
    ListNode(int x) : val(x), next(nullptr) {}
};
class Solution {
public:
    ListNode* mergeInBetween(ListNode* list1, int a, int b, ListNode* list2) {
        ListNode* prev_a = list1;
        for (int i = 1; i < a; ++i) prev_a = prev_a->next;
        ListNode* after_b = prev_a;
        for (int i = a; i <= b; ++i) after_b = after_b->next;
        ListNode* tail2 = list2;
        while (tail2->next) tail2 = tail2->next;
        prev_a->next = list2;
        tail2->next = after_b;
        return list1;
    }
};

Go

type ListNode struct {
    Val int
    Next *ListNode
}
func mergeInBetween(list1 *ListNode, a, b int, list2 *ListNode) *ListNode {
    prevA := list1
    for i := 1; i < a; i++ {
        prevA = prevA.Next
    }
    afterB := prevA
    for i := a; i <= b; i++ {
        afterB = afterB.Next
    }
    tail2 := list2
    for tail2.Next != nil {
        tail2 = tail2.Next
    }
    prevA.Next = list2
    tail2.Next = afterB
    return list1
}

Java

class ListNode {
    int val;
    ListNode next;
    ListNode(int x) { val = x; }
}
class Solution {
    public ListNode mergeInBetween(ListNode list1, int a, int b, ListNode list2) {
        ListNode prevA = list1;
        for (int i = 1; i < a; i++) prevA = prevA.next;
        ListNode afterB = prevA;
        for (int i = a; i <= b; i++) afterB = afterB.next;
        ListNode tail2 = list2;
        while (tail2.next != null) tail2 = tail2.next;
        prevA.next = list2;
        tail2.next = afterB;
        return list1;
    }
}

Kotlin

data class ListNode(var `val`: Int, var next: ListNode? = null)
class Solution {
    fun mergeInBetween(list1: ListNode, a: Int, b: Int, list2: ListNode): ListNode {
        var prevA = list1
        repeat(a - 1) { prevA = prevA.next!! }
        var afterB = prevA
        repeat(b - a + 1) { afterB = afterB.next!! }
        var tail2 = list2
        while (tail2.next != null) tail2 = tail2.next!!
        prevA.next = list2
        tail2.next = afterB
        return list1
    }
}

Python

class ListNode:
    def __init__(self, val: int, next: 'ListNode' = None):
        self.val = val
        self.next = next

def merge_in_between(list1: 'ListNode', a: int, b: int, list2: 'ListNode') -> 'ListNode':
    prev_a = list1
    for _ in range(a - 1):
        prev_a = prev_a.next
    after_b = prev_a
    for _ in range(b - a + 1):
        after_b = after_b.next
    tail2 = list2
    while tail2.next:
        tail2 = tail2.next
    prev_a.next = list2
    tail2.next = after_b
    return list1

Rust

struct ListNode {
    val: i32,
    next: Option<Box<ListNode>>,
}
impl Solution {
    pub fn merge_in_between(list1: Box<ListNode>, a: i32, b: i32, list2: Box<ListNode>) -> Box<ListNode> {
        let mut prev_a = &list1;
        for _ in 1..a {
            prev_a = prev_a.next.as_ref().unwrap();
        }
        let mut after_b = prev_a;
        for _ in a..=b {
            after_b = after_b.next.as_ref().unwrap();
        }
        let mut tail2 = &list2;
        while let Some(ref next) = tail2.next {
            tail2 = next;
        }
        // Unsafe pointer manipulation for brevity
        unsafe {
            let prev_a_mut = &mut *(prev_a as *const _ as *mut ListNode);
            let tail2_mut = &mut *(tail2 as *const _ as *mut ListNode);
            prev_a_mut.next = Some(list2);
            tail2_mut.next = Some(after_b.next.clone().unwrap());
        }
        list1
    }
}

TypeScript

class ListNode {
    val: number;
    next: ListNode | null;
    constructor(val: number, next: ListNode | null = null) {
        this.val = val;
        this.next = next;
    }
}
class Solution {
    mergeInBetween(list1: ListNode, a: number, b: number, list2: ListNode): ListNode {
        let prevA = list1;
        for (let i = 1; i < a; ++i) prevA = prevA.next!;
        let afterB = prevA;
        for (let i = a; i <= b; ++i) afterB = afterB.next!;
        let tail2 = list2;
        while (tail2.next) tail2 = tail2.next;
        prevA.next = list2;
        tail2.next = afterB;
        return list1;
    }
}

Complexity

• ⏰ Time complexity: O(n + m), traversing both lists once.
• 🧺 Space complexity: O(1), only pointers used.