203、移除链表元素
不带头节点
cpp
class Solution {
public:
ListNode* removeElements(ListNode* head, int val) {
while(head != NULL && head->val == val) {
ListNode* tem = head;
head = head->next;
delete tem;
}
ListNode* cur = head;
while(cur != NULL && cur->next != NULL) {
if(cur->next->val == val) {
ListNode* tem = cur->next;
cur->next = tem -> next;
delete tem;
}else {
cur = cur->next;
}
}
return head;
}
};
时间复杂度:O(n)
空间复杂度:O(1)
带头节点
cpp
class Solution {
public:
ListNode* removeElements(ListNode* head, int val) {
ListNode* dummy = new ListNode(0);
dummy -> next = head;
ListNode* cur = dummy;
while(cur->next != NULL) {
if(cur->next->val == val) {
ListNode* tem = cur->next;
cur->next = tem -> next;
delete tem;
}else {
cur = cur->next;
}
}
return dummy->next;
}
};
时间复杂度:O(n)
空间复杂度:O(1)
707、设计链表
注意链表结点的初始化、访问区间合不合法
cpp
class MyLinkedList {
public:
struct LinkedNode {
int val;
LinkedNode* next;
LinkedNode(int val):val(val), next(nullptr){}
};
MyLinkedList() {
dummyHead = new LinkedNode(0);
size = 0;
}
int get(int index) {
if(index <= -1 || index >= size) {
return -1;
}
LinkedNode* cur = dummyHead->next;
while(index--) {
cur = cur->next;
}
return cur->val;
}
void addAtHead(int val) {
LinkedNode* newNode = new LinkedNode(val);
newNode->next = dummyHead->next;
dummyHead->next = newNode;
size++;
}
void addAtTail(int val) {
LinkedNode* newNode = new LinkedNode(val);
LinkedNode* cur = dummyHead;
while(cur->next != NULL) {
cur = cur->next;
}
cur->next = newNode;
size++;
}
void addAtIndex(int index, int val) {
if(index <= -1 || index > size) {
return;
}
size++;
if(index == size) {
addAtTail(val);
return;
}
LinkedNode* newNode = new LinkedNode(val);
LinkedNode* cur = dummyHead;
while(index--) {
cur = cur->next;
}
newNode->next = cur->next;
cur->next = newNode;
}
void deleteAtIndex(int index) {
if(index <= -1 || index >= size) {
return ;
}
LinkedNode* cur = dummyHead;
while(index--) {
cur = cur->next;
}
LinkedNode* tem = cur->next;
cur->next = tem->next;
delete tem;
size--;
}
private:
LinkedNode* dummyHead;
int size;
};
206、反转链表
双指针:
用cur指针遍历旧链表,head指针指向新链表的第一个结点,用tem指针取出遍历到的结点,将其指向head作为新链表的第一个节点,更新head。
cpp
class Solution {
public:
ListNode* reverseList(ListNode* head) {
ListNode* cur = head;
head = NULL;
ListNode* tem = NULL;
while(cur != NULL) {
tem = cur;
cur = cur->next;
tem-> next = head;
head = tem;
}
return head;
}
};
时间复杂度:O(n)
空间复杂度:O(1)