题解 | #删除有序链表中重复的元素-II#

/**
 * struct ListNode {
 *  int val;
 *  struct ListNode *next;
 *  ListNode(int x) : val(x), next(nullptr) {}
 * };
 */
#include <unordered_map>
class Solution {
  public:
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     *
     * @param head ListNode类
     * @return ListNode类
     */
    ListNode* deleteDuplicates(ListNode* head) {
        // write code here

        std::unordered_map<int, int> countMap;

        // 统计每个元素的出现次数
        ListNode* current = head;
        while (current) {
            countMap[current->val]++;
            current = current->next;
        }

        // 创建新链表，只保留出现次数为1的元素
        ListNode* newHead = nullptr;
        ListNode* tail = nullptr;
        current = head;
        while (current) {
            if (countMap[current->val] == 1) {
                if (!newHead) {
                    newHead = tail = new ListNode(current->val);
                } else {
                    tail->next = new ListNode(current->val);
                    tail = tail->next;
                }
            }
            current = current->next;
        }

        return newHead;
    }

};

思路：哈希表统计数量，然后再找其中只出现一次的，加到新链表。尾插法，newhead和tail