leetcode203-remove-linked-list-elements_solution1.cpp

#include <iostream>
using namespace std;

/**
 * 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* removeElements(ListNode* head, int val) {
        ListNode *p = head;
        if(head == NULL)    /* 这里的条件不能是 head == NULL || head -> next == NULL */
            return p;

        // 迭代地删除开头的结点
        while(head != NULL && (head -> val) == val) /* 注意这里用while, 而不是if */
        {            
            head = head -> next;
        }

        while(p != NULL && (p -> next) != NULL)
        {
            // 删除非开头的结点
            if((p -> next) -> val == val)  // 此时 p 就是pre结点
            {
                p -> next = p->next->next;
            }
            else p = p -> next;
        }
        return head;
    }
};

// Test
int main()
{
    Solution sol;

    ListNode *head = new ListNode(7);
    head->next = new ListNode(7);
    head->next->next = new ListNode(3);
    head->next->next->next = new ListNode(7);
    head->next->next->next->next = NULL;
    int target = 7;

    ListNode *res = sol.removeElements(head, target);
    ListNode *p = res;
    while (p != NULL)
    {
        cout << p->val << endl;
        p = p->next;
    }

    return 0;
}