# 24. Swap Nodes in Pairs
Difficulty: Medium

## Solution

/**
*** Author: R-CO
*** E-mail: [e-mail]
*** Date: 2020-09-01
**/
#include <cstdlib>
#include <iostream>
using namespace std;

/**
 * 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) {}
 * };
 */
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) {
    ListNode* odd = nullptr;
    ListNode* even = nullptr;
    ListNode* pre_even = nullptr;
    ListNode* ptr = head;
    size_t count = 0;

    while (ptr != nullptr) {
      if ((++count & 0x1) == 0) {
        even = ptr;
        ptr = ptr->next;
        even->next = odd;
        odd->next = ptr;
        if (pre_even != nullptr) {
          pre_even->next = even;
        }
        pre_even = odd;
        if (count == 2) {
          head = even;
        }
      } else {
        odd = ptr;
        ptr = ptr->next;
      }
    }

    return head;
  }
};

void DisplayList(ListNode* head) {
  while (head != nullptr) {
    cout << head->val << " ";
    head = head->next;
  }
}

void ReleaseList(ListNode* head) {
  while (head != nullptr) {
    auto ptr = head;
    head = head->next;
    delete ptr;
  }
}

int main(int argc, char* argv[]) {
  /*
    1->2->3->4->5->6
    2->1->4->3->6->5
  */
  auto head = new ListNode(1);
  auto ptr = head;
  for (int i = 2; i <= 6; ++i) {
    ptr->next = new ListNode(i);
    ptr = ptr->next;
  }

  DisplayList(head);
  cout << endl;

  Solution solution;
  head = solution.swapPairs(head);

	DisplayList(head);
  cout << endl;

  ReleaseList(head);

  return EXIT_SUCCESS;
}

## Result

Success
Details
Runtime: 4 ms, faster than 59.11% of C++ online submissions for Swap Nodes in Pairs.
Memory Usage: 7.4 MB, less than 85.95% of C++ online submissions for Swap Nodes in Pairs.

###### tags: `LeetCode-Medium` `C++`