---
tags: leetcode
---

# [1669. Merge In Between Linked Lists](https://leetcode.com/problems/merge-in-between-linked-lists/)
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# My Solution
## The Key Idea for Solving This Coding Question
## C++ Code

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */

class Solution {
public:
    ListNode *mergeInBetween(ListNode *list1, int a, int b, ListNode *list2) {
        ListNode *left = list1, *right = list1;

        for (int i = 1; i < a; ++i) {
            left = left->next;
            right = right->next;
        }
        right = right->next;

        int diff = b - a + 1;
        while (diff > 0) {
            ListNode *x = right;
            right = right->next;
            delete x;
            --diff;
        }

        ListNode *list2Tail = list2;
        while (list2Tail->next != nullptr) {
            list2Tail = list2Tail->next;
        }

        left->next = list2;
        list2Tail->next = right;

        return list1;
    }
};

## Time Complexity
$O(n1 + n2)$
$n1$ is the number of nodes in the linked list referred by `list1`.
$n2$ is the number of nodes in the linked list referred by `list2`.
## Space Complexity
$O(1)$
# Miscellaneous
<!--
# Test Cases

[0,1,2,3,4,5]
3
4
[1000000,1000001,1000002]

[0,1,2,3,4,5,6]
2
5
[1000000,1000001,1000002,1000003,1000004]

-->