合并零之间的节点-力扣2181

合并零之间的节点-力扣2181

快慢指针同时向后遍历，快指针记录sum值，最后返回的结点值由慢指针保存。 为了能够在最后修改链表的尾巴，所以追踪的是快慢指针的next

/**
 * 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* mergeNodes(ListNode* head) {
        ListNode *slow = head, *fast = head;

        while(fast->next != nullptr){
            int sum = 0;
            while(fast->next != nullptr && fast->next->val == 0){ //快指针快速向后移动
                fast = fast->next;
            }

            while(fast->next != nullptr && fast->next->val != 0){
                sum += fast->next->val;
                fast = fast->next;
            }

            if(sum != 0){
                slow->next->val = sum;
                slow = slow->next;
            }

        }
        slow->next = nullptr;
        return head->next;
    }
};

时间复杂度：$O(n)$

空间复杂度：$O(1)$

错误写法，这种写法虽然追踪了当前指针，但是无法修改最后一个结点的尾巴。

class Solution {
public:
    ListNode* mergeNodes(ListNode* head) {
        ListNode *slow = head, *fast = head;

        while(fast != nullptr){
            int sum = 0;
            while(fast != nullptr && fast->val == 0){ //快指针快速向后移动
                fast = fast->next;
            }

            while(fast != nullptr && fast->val != 0){
                sum += fast->val;
                fast = fast->next;
            }

            if(sum != 0){
                slow->val = sum;
                slow = slow->next;
            }
        }

        slow = nullptr;

        return head;
    }
};