[LeetCode]80. Merge Two Sorted Lists合并两个排序链表
Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.
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解法1:递归。首先比较头节点大小,若l2->val>l1->val,则返回mergeTwoLists(l2,l1);否则(1)如果l1->next!=NULL,则比较l1->next->val与l2->val的大小,这样可以确定前两个元素的顺序,
/** * Definition for singly-linked list. * struct ListNode { * int val; * ListNode *next; * ListNode(int x) : val(x), next(NULL) {} * }; */ class Solution { public: ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) { if (l1 == NULL) return l2; if (l2 == NULL) return l1; if (l1->val <= l2->val) { if (l1->next != NULL && l1->next->val <= l2->val){ ListNode* node = mergeTwoLists(l1->next->next, l2); l1->next->next = node; } else if (l1->next != NULL && l1->next->val > l2->val) { ListNode* node = mergeTwoLists(l1->next, l2->next); l2->next = node; l1->next = l2; } else l1->next = l2; return l1; } else return mergeTwoLists(l2, l1); } };
解法2:迭代。新建一个指针help,然后遍历l1和l2,将二者中较小的链接到help后面。最后返回help的下一个节点作为头节点。注意两个链表可能长度不一样,要将长链表最后剩下的元素链接到新链表中。
/** * Definition for singly-linked list. * struct ListNode { * int val; * ListNode *next; * ListNode(int x) : val(x), next(NULL) {} * }; */ class Solution { public: ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) { if (l1 == NULL) return l2; if (l2 == NULL) return l1; ListNode* help = new ListNode(0); ListNode* head = help; while (l1 != NULL && l2 != NULL) { if (l1->val <= l2->val) { help->next = l1; l1 = l1->next; } else { help->next = l2; l2 = l2->next; } help = help->next; } if (l1 != NULL) help->next = l1; if (l2 != NULL) help->next = l2; return head->next; } };