# LeetCode - 1721. Swapping Nodes in a Linked List
### 題目網址：https://leetcode.com/problems/swapping-nodes-in-a-linked-list/
###### tags: `LeetCode` `Medium` `連結串列（Linked List）`

/*
    -LeetCode format-
    Problem: 1721. Swapping Nodes in a Linked List
    Difficulty: Medium
    
    by Inversionpeter
*/



/**
 * 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) {}
 * };
 */
static const auto Initialize = []{
    ios::sync_with_stdio(false); cin.tie(nullptr);
    return nullptr;
}();

class Solution {
public:
    ListNode* swapNodes(ListNode* head, int k) {
        int length = 0, fromRight;
        ListNode *now = head, *target1 = nullptr, *target2 = nullptr;
        do {
            ++length;
            now = now->next;
        } while (now);
        fromRight = length - k + 1;
        now = head;
        for (int i = 1; i <= length; ++i) {
            if (i == k)
                target1 = now;
            if (i == fromRight)
                target2 = now;
            now = now->next;
        }
        swap(target1->val, target2->val);
        return head;
    }
};