Reverse Linked List Groups - Amazon Top Interview Questions

Problem Statement :

Given a singly linked list node, and an integer k, reverse every k contiguous group of nodes.


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

Example 1


node = [0, 1, 2, 3]
k = 2


[1, 0, 3, 2]

Example 2


node = [0, 1, 2, 3]
k = 3


[2, 1, 0, 3]

Solution :


                        Solution in C++ :

 * class LLNode {
 *     public:
 *         int val;
 *         LLNode *next;
 * };
LLNode* solve(LLNode* head, int k) {
    if (!head || !head->next) return head;
    stack<LLNode*> st;
    int count = k;
    while (head && count) {
        LLNode* tmp = head;
        head = head->next;
        tmp->next = NULL;
    LLNode *tmp_head_node =, *it_node =;
    while (st.size()) {
        it_node->next =;
        it_node = it_node->next;
    if (head) {
        it_node->next = solve(head, k);
    return tmp_head_node;

                        Solution in Python : 
# class LLNode:
#     def __init__(self, val, next=None):
#         self.val = val
# = next
class Solution:
    def solve(self, head, k):
        cur = head
        temp = None
        prev = None
        count = 0
        while cur != None and count < k:
            temp =
   = prev
            prev = cur
            cur = temp
            count += 1
        if temp is not None:
   = self.solve(temp, k)
        return prev

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