**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. Constraints n ≤ 100,000 where n is the number of nodes in node Example 1 Input node = [0, 1, 2, 3] k = 2 Output [1, 0, 3, 2] Example 2 Input node = [0, 1, 2, 3] k = 3 Output [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;
st.push(tmp);
--count;
}
LLNode *tmp_head_node = st.top(), *it_node = st.top();
st.pop();
while (st.size()) {
it_node->next = st.top();
st.pop();
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
# self.next = next
class Solution:
def solve(self, head, k):
cur = head
temp = None
prev = None
count = 0
while cur != None and count < k:
temp = cur.next
cur.next = prev
prev = cur
cur = temp
count += 1
if temp is not None:
head.next = self.solve(temp, k)
return prev
```

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