链表的常考题目
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode* reverseList(ListNode* head) {
ListNode * new_head = nullptr;
while(head)
{
ListNode* next = head->next;
head->next = new_head;
new_head = head;
head = next;
}
return new_head;
}
};
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode* reverseBetween(ListNode* head, int m, int n) {
int change_len = n-m+1;
ListNode *pre_head = nullptr;
ListNode *result = head;
while(head && --m)
{
pre_head = head;
head = head->next;
}
ListNode* tail = head;
ListNode* new_head = nullptr;
while(head && change_len)
{
ListNode * next = head->next;
head->next = new_head;
new_head = head;
head = next;
change_len--;
}
tail->next = head;
if(pre_head!=nullptr)
{
pre_head->next = new_head;
}
else
{
result = new_head;
}
return result;
}
};
方法一：利用哈希表
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode *getIntersectionNode(ListNode *headA, ListNode *headB) {
unordered_set<ListNode*> s;
while(headB)
{
s.insert(headB);
headB = headB->next;
}
while(headA)
{
if(s.count(headA))
return headA;
headA = headA->next;
}
return nullptr;
}
};
方法二：双指针
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
int get_list_length(ListNode *head)
{
int len = 0;
while(head)
{
len++;
head = head->next;
}
return len;
}
ListNode * movehead(int longlen, int shortlen, ListNode* head)
{
int dif = longlen-shortlen;
while(dif)
{
head = head->next;
dif--;
}
return head;
}
class Solution {
public:
ListNode *getIntersectionNode(ListNode *headA, ListNode *headB) {
int la = get_list_length(headA);
int lb = get_list_length(headB);
if (la>lb){
headA = movehead(la,lb,headA);
}
else
{
headB = movehead(lb,la,headB);
}
while(headA && headB)
{
if(headA==headB)
{
return headA;
}
headA = headA->next;
headB = headB->next;
}
return nullptr;
}
};
方法一：利用set
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
bool hasCycle(ListNode *head) {
set<ListNode*>s;
while(head)
{
if (s.count(head))
return true;
s.insert(head);
head = head->next;
}
return false;
}
};
方法二：快慢指针
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
bool hasCycle(ListNode *head) {
if(head==nullptr || head->next == nullptr)
return false;
ListNode *slow = head;
ListNode *fast = head->next;
while(slow != fast)
{
if (fast==nullptr || fast->next==nullptr)
return false;
slow = slow->next;
fast = fast->next->next;
}
return true;
}
};
方法一：利用set
class Solution {
public:
ListNode *detectCycle(ListNode *head) {
unordered_set<ListNode *> st;
while(head!=NULL){
if(st.count(head)) return head;
st.insert(head);
head=head->next;
}
return NULL;
}
};
方法二：快慢指针
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode *detectCycle(ListNode *head) {
ListNode *fast = head, *slow = head;
while (fast and fast->next) { //写的时候注意 fast == slow 不要写在这儿，因为刚开始走的时候两个都是 head
slow = slow->next;
fast = fast->next->next;
if (slow == fast) {
break;
}
}
if (fast == NULL || fast->next == NULL) {
return NULL;
}
ListNode *offset = head;
while (offset != fast) {
offset = offset->next;
fast = fast->next;
}
return offset;
}
};
方法：利用临时头结点
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode* partition(ListNode* head, int x) {
ListNode lesshead(0);
ListNode morehead(0);
ListNode *less = &lesshead;
ListNode *more = &morehead;
while(head)
{
if (head->val < x)
{
less->next = head;
less = less->next;
}
else
{
more->next = head;
more = more->next;
}
head = head->next;
}
less->next = morehead.next;
more->next= nullptr;
return lesshead.next;
}
};
方法一：利用一个哈希表，存储一个地址到节点位置的map
/*
// Definition for a Node.
class Node {
public:
int val;
Node* next;
Node* random;
Node(int _val) {
val = _val;
next = NULL;
random = NULL;
}
};
*/
class Solution {
public:
Node* copyRandomList(Node* head) {
map<Node*, int> node_map;
vector< Node*> node_vec;
Node *ptr = head;
int i= 0;
while(ptr)
{
node_vec.push_back(new Node(ptr->val));
node_map[ptr] = i;
ptr = ptr->next;
i++;
}
node_vec.push_back(0);
ptr = head;
i = 0;
while(ptr)
{
node_vec[i]->next = node_vec[i+1];
if(ptr->random)
{
int id = node_map[ptr->random];
node_vec[i]->random = node_vec[id];
}
ptr = ptr->next;
i++;
}
return node_vec[0];
}
};
方法二：不使用辅助空间的情况下实现O(N)的时间效率
/*
// Definition for a Node.
class Node {
public:
int val;
Node* next;
Node* random;
Node(int _val) {
val = _val;
next = NULL;
random = NULL;
}
};
*/
void colneNodes(Node* head)
{
Node* ptr = head;
while(ptr)
{
Node* pclone = new Node(ptr->val);
pclone->next = ptr->next;
pclone->random = nullptr;
ptr->next = pclone;
ptr = pclone->next;
}
}
void connectRandom(Node* head)
{
Node *ptr = head;
while(ptr)
{
Node * pclone = ptr->next;
if (ptr->random != nullptr)
{
pclone->random = ptr->random->next;
}
ptr = pclone->next;
}
}
Node* Reconnect(Node* head)
{
Node *ptr = head;
Node *pcloneHead = nullptr;
Node *pcloneNode = nullptr;
if (ptr)
{
pcloneHead = pcloneNode = ptr->next;
ptr->next = pcloneNode->next;
ptr = ptr->next;
}
while(ptr)
{
pcloneNode->next = ptr->next;
pcloneNode = pcloneNode->next;
ptr->next = pcloneNode->next;
ptr = ptr->next;
}
return pcloneHead;
}
class Solution {
public:
Node* copyRandomList(Node* head) {
colneNodes(head);
connectRandom(head);
return Reconnect(head);
}
};
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode* mergeTwoList(ListNode *a, ListNode *b)
{
if (a == nullptr || b == nullptr)
return a ? a : b;
ListNode *ptr
}
ListNode* mergeKLists(vector<ListNode*>& lists) {
vector<ListNode *> v;
for (int i=0;i<lists.size();i++)
{
ListNode* head = lists[i];
while(head)
{
v.push_back(head);
head = head->next;
}
}
if(v.size()==0)
return nullptr;
sort(v.begin(),v.end(),cmp);
for (int i=1;i<v.size();i++)
{
v[i-1]->next = v[i];
}
v[v.size()-1]->next = nullptr;
return v[0];
}
};
方法一：排序后相连
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
bool cmp(const ListNode* a, const ListNode* b)
{
return a->val < b->val;
}
class Solution {
public:
ListNode* mergeKLists(vector<ListNode*>& lists) {
vector<ListNode *> v;
for (int i=0;i<lists.size();i++)
{
ListNode* head = lists[i];
while(head)
{
v.push_back(head);
head = head->next;
}
}
if(v.size()==0)
return nullptr;
sort(v.begin(),v.end(),cmp);
for (int i=1;i<v.size();i++)
{
v[i-1]->next = v[i];
}
v[v.size()-1]->next = nullptr;
return v[0];
}
};
方法二：分治后相连
/**
* 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==nullptr)
return l2;
if (l2==nullptr)
return l1;
ListNode * first = new ListNode(0);
ListNode * head = first;
while (l1 != nullptr && l2 != nullptr)
{
if(l1->val <l2->val)
{
first->next = l1;
l1 = l1->next;
}
else
{
first->next = l2;
l2 = l2->next;
}
first = first->next;
}
first->next = l1 == nullptr ? l2 : l1;
return head->next;
}
ListNode* merge(vector<ListNode*>& lists, int l, int r)
{
if (l==r)
return lists[l];
if(l>r)
return nullptr;
int mid = (l + r) >> 1;
return mergeTwoLists(merge(lists, l, mid), merge(lists, mid + 1, r));
}
ListNode* mergeKLists(vector<ListNode*>& lists) {
return merge(lists, 0, lists.size() - 1);
}
};
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