Insert Into Linked List - Facebook Top Interview Questions

Problem Statement :

You are given a singly linked list head as well as integers pos and val. Insert a new node with value val before index pos of head.


1 ≤ n ≤ 100,000 where n is the number of nodes in head

0 ≤ pos ≤ n

Example 1


head = [1, 3, 5, 7]

pos = 2

val = 9


[1, 3, 9, 5, 7]

Example 2


head = [1]

pos = 0

val = 3


[3, 1]

Example 3


head = [2]

pos = 1

val = 5


[2, 5]

Solution :

                        Solution in C++ :

LLNode* solve(LLNode* head, int pos, int val) {
    auto dum = new LLNode(-1, head), prev = dum;
    while (pos--) {
        prev = head;
        head = head->next;
    LLNode* temp = new LLNode(val, head);
    prev->next = temp;
    return dum->next;

                        Solution in Java :

import java.util.*;

 * class LLNode {
 *   int val;
 *   LLNode next;
 * }
class Solution {
    public LLNode solve(LLNode head, int pos, int val) {
        if (pos == 0) {
            LLNode ans = new LLNode(val, head);
            return ans;
        LLNode temp = head;
        while (pos > 1) {
            temp =;
        } = new LLNode(val,;
        return head;

                        Solution in Python : 
class Solution:
    def solve(self, head, pos, val):
        new = LLNode(val)

        if pos == 0:
   = head
            return new

        temp = head
        while temp and pos != 1:
            temp =
            pos -= 1 = = new

        return head

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