链表反转

迭代版本

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

递归版本

/**
 * 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* reverseList(ListNode* head) {
        if(!head || !head->next) return head;
        auto newhead = reverseList(head->next);
        head->next->next = head;
        head->next = nullptr;
        return newhead;
    }
};

/**
 * 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* reverseBetween(ListNode* head, int left, int right) {
        auto dummy = new ListNode(-1);
        dummy->next = head;
        auto a = dummy;
        for(int i = 0; i < left-1; i++) a = a->next;
        auto b = a->next, c = b->next;
        for(int i = 0; i < right-left; i++) {
            auto d = c->next;
            c->next = b;
            b = c, c = d;
        }
        a->next->next = c;
        a->next = b;
        return dummy->next;
    }
};

LeetCode 两数相加

/**
 * 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* addTwoNumbers(ListNode* l1, ListNode* l2) {
        auto dummy = new ListNode(-1);
        auto tmp = dummy;
        int carry = 0;
        while(l1 || l2) {
            int val = (l1 ? l1->val : 0) + (l2 ? l2->val : 0) + carry;
            tmp->next = new ListNode(val % 10);
            carry = val / 10;
            tmp = tmp->next;
            l1 = l1 ? l1->next : nullptr;
            l2 = l2 ? l2->next : nullptr;
        }
        if(carry) tmp->next = new ListNode(1);
        return dummy->next;
    }
};

LeetCode 445. 两数相加 II

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* reverse(ListNode* head) {
        auto a = head, b = head->next;
        while(b) {
            auto c = b->next;
            b->next = a;
            a = b, b = c;
        }
        head->next = nullptr;
        return a;
    }

    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
        l1 = reverse(l1), l2 = reverse(l2);
        auto dummy = new ListNode(-1);
        int t = 0;
        while(l1 || l2 || t) {
            if(l1) t += l1->val, l1 = l1->next;
            if(l2) t += l2->val, l2 = l2->next;
            auto cur = new ListNode(t % 10);
            t /= 10;
            cur->next = dummy->next;
            dummy->next = cur;
        }
        return dummy->next;
    }
};

LeetCode 19. 删除链表的倒数第 N 个结点

/**
 * 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* removeNthFromEnd(ListNode* head, int n) {
        auto dummy = new ListNode(-1);
        dummy->next = head;
        int m = 0;
        for(auto p = head; p; p = p->next) m++;
        n = m - n;
        auto tmp = dummy;
        for(int i = 0; i < n; i++) tmp = tmp->next;
        tmp->next = tmp->next->next;
        return dummy->next;
    }
};

快慢指针

/**
 * 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* removeNthFromEnd(ListNode* head, int n) {
        auto dummy = new ListNode(-1);
        dummy->next = head;
        auto fast = dummy, slow = dummy;
        for(int i = 0; i < n; i++) fast = fast->next;
        while(fast->next) {
            fast = fast->next;
            slow = slow->next;
        }
        slow->next = slow->next->next;
        return dummy->next;
    }
};

LeetCode 21. 合并两个有序链表

/**
 * 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) {
        auto dummy = new ListNode(-1), tail = dummy;
        while(l1 && l2) {
            if(l1->val < l2->val) {
                tail = tail->next = l1;
                l1 = l1->next;
            }else {
                tail = tail->next = l2;
                l2 = l2->next;
            }
        }
        if(l1) tail->next = l1;
        if(l2) tail->next = l2;
        return dummy->next;
    }
};

LeetCode 23. 合并K个升序链表

/**
 * 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:
    struct cmp {
        bool operator() (ListNode* a, ListNode* b) {
            return a->val > b->val;
        }
    };

    ListNode* mergeKLists(vector<ListNode*>& lists) {
        priority_queue<ListNode*, vector<ListNode*>, cmp> heap;
        auto dummy = new ListNode(-1), tail = dummy;
        for(auto l : lists) if(l) heap.push(l);

        while(heap.size()) {
            auto t = heap.top();
            heap.pop();

            tail = tail->next = t;
            if(t->next) heap.push(t->next);
        }
        return dummy->next;
    }
};