1721

/**
 * 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* swapNodes(ListNode* head, int k) {
        ListNode *left, *right, *curr;
        int len=0;
        curr = head;
        while(curr){
            len++;
            if(len == k) left=curr;
            curr = curr->next;
        }
        curr=head;
        for(int i=1; i<=len; i++){
            if(len-k+1 == i) right = curr;
            curr = curr->next;
        }
        swap(left->val, right->val);
        return head;
    }
};