C++實現LeetCode(203.移除鏈表元素)

[LeetCode] 203.Remove Linked List Elements 移除鏈表元素

Remove all elements from a linked list of integers that have value val.

Example
Given: 1 –> 2 –> 6 –> 3 –> 4 –> 5 –> 6, val = 6
Return: 1 –> 2 –> 3 –> 4 –> 5

Credits:
Special thanks to @mithmatt for adding this problem and creating all test cases.

這道移除鏈表元素是鏈表的基本操作之一,沒有太大的難度,就是考察瞭基本的鏈表遍歷和設置指針的知識點,我們隻需定義幾個輔助指針,然後遍歷原鏈表,遇到與給定值相同的元素,將該元素的前後連個節點連接起來,然後刪除該元素即可,要註意的是還是需要在鏈表開頭加上一個dummy node,具體實現參見代碼如下:

解法一:

class Solution {
public:
    ListNode* removeElements(ListNode* head, int val) {
        ListNode *dummy = new ListNode(-1), *pre = dummy;
        dummy->next = head;
        while (pre->next) {
            if (pre->next->val == val) {
                ListNode *t = pre->next;
                pre->next = t->next;
                t->next = NULL;
                delete t;
            } else {
                pre = pre->next;
            }
        }
        return dummy->next;
    }
};

如果隻是為瞭通過OJ,不用寫的那麼嚴格的話,下面這種方法更加簡潔,當判斷下一個結點的值跟給定值相同的話,直接跳過下一個結點,將next指向下下一個結點,而根本不斷開下一個結點的next,更不用刪除下一個結點瞭。最後還要驗證頭結點是否需要刪除,要的話直接返回下一個結點,參見代碼如下:

解法二:

class Solution {
public:
    ListNode* removeElements(ListNode* head, int val) {
        if (!head) return NULL;
        ListNode *cur = head;
        while (cur->next) {
            if (cur->next->val == val) cur->next = cur->next->next;
            else cur = cur->next;
        }
        return head->val == val ? head->next : head;
    }
};

我們也可以用遞歸來解,寫法很簡潔,通過遞歸調用到鏈表末尾,然後回來,需要要刪的元素,將鏈表next指針指向下一個元素即可:

解法三:

class Solution {
public:
    ListNode* removeElements(ListNode* head, int val) {
        if (!head) return NULL;
        head->next = removeElements(head->next, val);
        return head->val == val ? head->next : head;
    }
};

類似題目:

Remove Element

Delete Node in a Linked List

參考資料:

https://leetcode.com/problems/remove-linked-list-elements/

https://leetcode.com/problems/remove-linked-list-elements/discuss/57324/AC-Java-solution

https://leetcode.com/problems/remove-linked-list-elements/discuss/57306/3-line-recursive-solution

https://leetcode.com/problems/remove-linked-list-elements/discuss/57331/Accepted-7-line-clean-java-solution

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