Leetcode[206] 反转链表
题目描述
反转一个单链表
示例:
输入: 1->2->3->4->5->NULL
输出: 5->4->3->2->1->NULL
题解思路
迭代思路
- 边界条件:当只有零个或一个节点时,返回头节点指针即可;
- 建立以第一个节点新的链表头,新表头 next 为 NULL;
- 从第二个节点遍历链表,从头部插入新的链表,临时指针 temp 存放 next 节点指针;
递归思路
- 终止条件:当前节点或者下一个节点为NULL;
- 在函数内部递归改变节点指向:head -> next -> next = head;
AC代码
迭代算法:
struct ListNode* reverseList(struct ListNode* head){
// 边界条件,当 head == NULL 或 head -> next == NULL, return head
if(head == NULL || head -> next == NULL) {
return head;
}
struct ListNode *p = head -> next;
struct ListNode *newhead = head;
newhead -> next = NULL;
struct ListNode *temp = NULL;
while(p != NULL) {
temp = p -> next;
p -> next = newhead;
newhead = p;
p = temp;
}
return newhead;
}
递归算法:
struct ListNode* reverseList(struct ListNode* head){
if(head == NULL || head -> next == NULL) {
return head;
}
struct ListNode *p = reverseList(head -> next);
head -> next -> next = head;
head -> next = NULL;
return p;
}
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