每日一題_連結清單專題

前言

date: 9.21

中秋假節

彙總: 每日一題系列_算法提升

合并兩個有序連結清單

題目: leetcode 21.合并兩個有序連結清單

遞歸版本：

/**
 * 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* mergeTwoLists(ListNode* l1, ListNode* l2) {
        if (l1 == nullptr)
            return l2;
        if (l2 == nullptr)
            return l1;
        if (l1->val < l2->val) {
            l1->next = mergeTwoLists(l1->next, l2);
            return l1;
        }
        else {
            l2->next = mergeTwoLists(l1, l2->next);
            return l2;
        }
    }
};
           

非遞歸版本:

/**
 * 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* mergeTwoLists(ListNode* l1, ListNode* l2) {
        if (l1 == nullptr)
            return l2;
        if (l2 == nullptr)
            return l1;
        
        ListNode* head = new ListNode();
        ListNode* p1 = l1, *p2 = l2, *p = head;

        while (p1 != nullptr && p2 != nullptr) {
            if (p1->val < p2->val) {
                p->next = p1;
                p1 = p1->next;
                p = p->next;
            } else {
                p->next = p2;
                p2 = p2->next;
                p = p->next;
            }
        }
        
        while(p1 != nullptr) {
            p->next = p1;
            p1 = p1->next;
            p = p->next;
        }
        while(p2 != nullptr) {
            p->next = p2;
            p2 = p2->next;
            p = p->next;
        }

        p = head->next;
        delete head;
        return p;
    }
};
           

反轉連結清單

題目： leetcode 206.反轉連結清單

開多幾個輔助指針就好啦

/**
 * 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* reverseList(ListNode* head) {
        ListNode* pre = nullptr, *cur = head, *next;
        while(cur) {
            next = cur->next;
            cur->next = pre;
            pre = cur;
            cur = next;
        }
        return pre;
    }
};
           

回文連結清單

題目來源： leetcode 234回文連結清單

用個vec存一下吧。

/**
 * 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:
    bool isPalindrome(ListNode* head) {
        vector<int> vec;

        ListNode* p = head;
        while(p) {
            vec.push_back(p->val);
            p = p->next;
        }

        int n = vec.size();
        for (int i = 0; i < n / 2; i++)
            if (vec[i] != vec[n - i - 1])
                return false;
        return true;
    }
};