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LeetCode 1721. Swapping Nodes in a Linked List

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题目

原题在此

解析

这次的题是交换val, 不是交换node, 所以更简单.
两个指针p1, p2, p1先走k步, 然后p1和p2一起走, 当p1走到头的时候, p2就是倒数第k个了

代码

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/**
 * 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 holder(0, head);
        ListNode* foo = head, *bar = head;
        for(int i = 0; i < k; i++) {
            foo = foo -> next;
        }
        while(foo){
            foo = foo -> next;
            bar = bar -> next;
        }
        foo = &holder;
        for(int i = 0; i < k; i++) {
            foo = foo -> next;
        }
        swap(foo->val, bar->val);
        return holder.next;
    }
};