LeetCode-in-Cpp
24. Swap Nodes in Pairs
Medium
Given a linked list, swap every two adjacent nodes and return its head. You must solve the problem without modifying the values in the list’s nodes (i.e., only nodes themselves may be changed.)
Example 1:
Input: head = [1,2,3,4]
Output: [2,1,4,3]
Example 2:
Input: head = []
Output: []
Example 3:
Input: head = [1]
Output: [1]
Constraints:
- The number of nodes in the list is in the range
[0, 100]. 0 <= Node.val <= 100
Solution
/**
* 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* swapPairs(ListNode* head) {
if (head == nullptr) {
return nullptr;
}
int len = getLength(head);
return reverse(head, len);
}
private:
int getLength(ListNode* curr) {
int cnt = 0;
while (curr != nullptr) {
cnt++;
curr = curr->next;
}
return cnt;
}
ListNode* reverse(ListNode* head, int len) {
// base case
if (len < 2) {
return head;
}
ListNode* curr = head;
ListNode* prev = nullptr;
ListNode* next = nullptr;
for (int i = 0; i < 2; i++) {
// reverse linked list code
next = curr->next;
curr->next = prev;
prev = curr;
curr = next;
}
head->next = reverse(curr, len - 2);
return prev;
}
};