按照frequency来排序，总共27题
1、2. Add Two Numbers https://leetcode.com/problems/add-two-numbers/#/description
两个链表相加，注意就是进位问题。
/**
* Definition for singly-linked list.
* public class ListNode {
* int val;
* ListNode next;
* ListNode(int x) { val = x; }
* }
*/
public class Solution {
public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
int flag = 0;
ListNode result = new ListNode(0);
ListNode node = result;
while (l1 != null || l2 != null){
int val = flag;
if (l1 != null){
val += l1.val;
l1 = l1.next;
}
if (l2 != null){
val += l2.val;
l2 = l2.next;
}
if (val > 9){
val -= 10;
flag = 1;
} else {
flag = 0;
}
node.next = new ListNode(val);
node = node.next;
}
if (flag == 1){
node.next = new ListNode(1);
}
return result.next;
}
}
2、237. Delete Node in a Linked List https://leetcode.com/problems/delete-node-in-a-linked-list/#/description
删除链表中的节点。只给需要删除的节点。很简单，两行代码就搞定了。
/**
* Definition for singly-linked list.
* public class ListNode {
* int val;
* ListNode next;
* ListNode(int x) { val = x; }
* }
*/
public class Solution {
public void deleteNode(ListNode node) {
node.val = node.next.val;
node.next = node.next.next;
}
}
3、206. Reverse Linked List https://leetcode.com/problems/reverse-linked-list/#/description
反转链表。
/**
* Definition for singly-linked list.
* public class ListNode {
* int val;
* ListNode next;
* ListNode(int x) { val = x; }
* }
*/
public class Solution {
public ListNode reverseList(ListNode head) {
if (head == null || head.next == null){
return head;
}
ListNode pre = head;
ListNode node = head.next;
head.next = null;
while (node.next != null){
ListNode flag = node.next;
node.next = head;
head = node;
node = flag;
}
node.next = head;
return node;
}
}
4、148. Sort List https://leetcode.com/problems/sort-list/#/description
链表排序。要求nlogn的时间复杂度和常数的空间复杂度。那么应用归并排序。
/**
* Definition for singly-linked list.
* public class ListNode {
* int val;
* ListNode next;
* ListNode(int x) { val = x; }
* }
*/
public class Solution {
public ListNode sortList(ListNode head) {
if (head == null || head.next == null){
return head;
}
ListNode pre = null;
ListNode slow = head;
ListNode fast = head;
while (fast != null && fast.next != null){
pre = slow;
slow = slow.next;
fast = fast.next.next;
}
pre.next = null;
ListNode l1 = sortList(head);
ListNode l2 = sortList(slow);
return sortList(l1, l2);
}
public ListNode sortList(ListNode node1, ListNode node2){
ListNode node = new ListNode(0);
ListNode nn = node;
while (node1 != null && node2 != null){
if (node1.val < node2.val){
nn.next = node1;
node1 = node1.next;
} else {
nn.next = node2;
node2 = node2.next;
}
nn = nn.next;
}
while (node1 != null){
nn.next = node1;
nn = nn.next;
node1 = node1.next;
}
while (node2 != null){
nn.next = node2;
nn = nn.next;
node2 = node2.next;
}
return node.next;
}
}
5、141. Linked List Cycle https://leetcode.com/problems/linked-list-cycle/#/description
判断链表是否有环。
一个快，一个慢，判断是否会走到一起（如果成环一定可以走到一起）。
/**
* Definition for singly-linked list.
* class ListNode {
* int val;
* ListNode next;
* ListNode(int x) {
* val = x;
* next = null;
* }
* }
*/
public class Solution {
public boolean hasCycle(ListNode head) {
if (head == null || head.next == null){
return false;
}
ListNode slow = head.next;
ListNode fast = head.next.next;
while (fast != null && fast.next != null && slow != fast){
fast = fast.next.next;
slow = slow.next;
}
return (fast == null || fast.next == null) ? false : true;
}
}
6、328. Odd Even Linked List https://leetcode.com/problems/odd-even-linked-list/#/description
将链表分为偶数和奇数两块，奇数放在前面，偶数放在后面。
刚开始看错题目，说的是将第偶数个放在一起，第奇数个放在一起，并非链表中的数值。
/**
* Definition for singly-linked list.
* public class ListNode {
* int val;
* ListNode next;
* ListNode(int x) { val = x; }
* }
*/
public class Solution {
public ListNode oddEvenList(ListNode head) {
if (head == null || head.next == null){
return head;
}
ListNode odd = head, even = head.next, evenhead = even;
while (even != null && even.next != null){
odd.next = odd.next.next;
even.next = even.next.next;
odd = odd.next;
even = even.next;
}
odd.next = evenhead;
return head;
}
}
7、21. Merge Two Sorted Lists 链表合并 https://leetcode.com/problems/merge-two-sorted-lists/#
简单的两路归并。
/**
* Definition for singly-linked list.
* public class ListNode {
* int val;
* ListNode next;
* ListNode(int x) { val = x; }
* }
*/
public class Solution {
public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
ListNode head = new ListNode(0), node = head;;
while (l1 != null && l2 != null){
if (l1.val > l2.val){
node.next = l2;
l2 = l2.next;
} else {
node.next = l1;
l1 = l1.next;
}
node = node.next;
}
if (l1 != null){
node.next = l1;
} else if (l2 != null){
node.next = l2;
}
return head.next;
}
}
8、160. Intersection of Two Linked Lists返回两个链表的第一个交点。 https://leetcode.com/problems/intersection-of-two-linked-lists/#/description
a、找出长度差，然后长的链表先行长度差个，然后依次比较，找出交点（没有就是null）。
b、用hash
/**
* Definition for singly-linked list.
* public class ListNode {
* int val;
* ListNode next;
* ListNode(int x) {
* val = x;
* next = null;
* }
* }
*/
public class Solution {
public ListNode getIntersectionNode(ListNode headA, ListNode headB) {
int len1 = 0, len2 = 0;
ListNode node = headA;
while (node != null){
node = node.next;
len1++;
}
node = headB;
while (node != null){
node = node.next;
len2++;
}
if (len1 > len2){
for (int i = 0; i < len1 - len2; i++){
headA = headA.next;
}
} else {
for (int i = 0; i < len2 - len1; i++){
headB = headB.next;
}
}
while (headA != headB){
headA = headA.next;
headB = headB.next;
}
return headA;
}
}
另外，可以这样解。
public ListNode getIntersectionNode(ListNode headA, ListNode headB) {
//boundary check
if(headA == null || headB == null) return null;
ListNode a = headA;
ListNode b = headB;
//if a & b have different len, then we will stop the loop after second iteration
while( a != b){
//for the end of first iteration, we just reset the pointer to the head of another linkedlist
a = a == null? headB : a.next;
b = b == null? headA : b.next;
}
return a;
}
9、138. Copy List with Random Pointer 拷贝随机链表 https://leetcode.com/problems/copy-list-with-random-pointer/#/description
a、hash
b、在每一个链表后面添加一个链表。
/**
* Definition for singly-linked list with a random pointer.
* class RandomListNode {
* int label;
* RandomListNode next, random;
* RandomListNode(int x) { this.label = x; }
* };
*/
public class Solution {
public RandomListNode copyRandomList(RandomListNode head) {
if (head == null){
return null;
}
RandomListNode node = head, next;
while (node != null){
next = node.next;
node.next = new RandomListNode(node.label);
node.next.next = next;
node = next;
}
node = head;
while (node != null){
if (node.random != null){
node.next.random = node.random.next;
}
node = node.next.next;
}
RandomListNode newHead = new RandomListNode(0), result = newHead;
node = head;
while (node != null){
next = node.next.next;
newHead.next = node.next;
newHead = newHead.next;
node.next = next;
node = node.next;
}
return result.next;
}
}
10、23. Merge k Sorted Lists k个链表的合并。 https://leetcode.com/problems/merge-k-sorted-lists/#/description
k路归并，利用优先级队列PriorityQueue
/**
* Definition for singly-linked list.
* public class ListNode {
* int val;
* ListNode next;
* ListNode(int x) { val = x; }
* }
*/
public class Solution {
public ListNode mergeKLists(ListNode[] lists) {
ListNode head = new ListNode(0), node = head;
int size = lists.length;
if (size == 0){
return null;
}
Comparator<ListNode> comparator = new Comparator<ListNode>(){
public int compare(ListNode o1, ListNode o2){
if (o1.val > o2.val){
return 1;
} else if (o1.val < o2.val){
return -1;
} else {
return 0;
}
}
};
PriorityQueue<ListNode> queue = new PriorityQueue(size, comparator);
for (int i = 0; i < size; i++){
if (lists[i] != null)
queue.add(lists[i]);
}
while (!queue.isEmpty()){
node.next = queue.poll();
node = node.next;
if (node.next != null){
queue.add(node.next);
}
}
return head.next;
}
}
11、234. Palindrome Linked List 判断是否是回文链表 https://leetcode.com/problems/palindrome-linked-list/#/solutions
（123，123）是回文，（123，321）不是。
由于没有时间了，写上思路：a、利用栈，时间O(n),空间O(N);
b、找到中点（fast、slow一快一慢来判断），后半部分反转。然后判断。
12、147. Insertion Sort List 链表插入排序 https://leetcode.com/problems/insertion-sort-list/#/description
没什么技巧和难点。
13、142. Linked List Cycle II 找出链表循环的起点 https://leetcode.com/problems/linked-list-cycle-ii/#/description
与第五题也就是141相同，相遇的点就是循环的起点（数学方法证明）。
14、19. Remove Nth Node From End of List 删除倒数第n个节点 https://leetcode.com/problems/remove-nth-node-from-end-of-list/#/description
fast先走n个节点，然后slow再走，直到 fast == null为止，跳过slow.next即可。
这里需要了解n的范围，会不会超过长度，如果超过了长度要怎么办。
15、143. Reorder List 将链表按照固定方式改写。 https://leetcode.com/problems/reorder-list/#/description
Given a singly linked list L: L0→L1→…→Ln-1→Ln,
reorder it to: L0→Ln→L1→Ln-1→L2→Ln-2→…
1、用list或者数组记录
2、用双向队列（Deque）
3、链表分成两半，然后后一半反转，然后合并两个链表。
16、24. Swap Nodes in Pairs 节点两两交换 https://leetcode.com/problems/swap-nodes-in-pairs/#/description
a、递归实现
b、非递归
17、203. Remove Linked List Elements 删除某个值 https://leetcode.com/problems/remove-linked-list-elements/#/description
遍历直接删除就行了
18、83. Remove Duplicates from Sorted List 删除重复的数 https://leetcode.com/problems/remove-duplicates-from-sorted-list/#/description
也是递归、非递归实现。
19、109. Convert Sorted List to Binary Search Tree 排序好的链表建树 https://leetcode.com/problems/convert-sorted-list-to-binary-search-tree/#/description
递归实现，二分查找。
20、92. Reverse Linked List II 链表的部分反转 https://leetcode.com/problems/reverse-linked-list-ii/#/description
21、86. Partition List 划分链表的值 https://leetcode.com/problems/partition-list/#/description
给定一个值，把小于x的值放在左边，大于等于x的值放在右边，保持相对顺序。
没什么难点。
22、25. Reverse Nodes in k-Group 部分反转 https://leetcode.com/problems/reverse-nodes-in-k-group/#/description
也是递归和非递归的实现。
23、61. Rotate List 旋转链表 https://leetcode.com/problems/rotate-list/#/description
从倒数第k个位置开始旋转一下链表。
需要判断k是否大于len。
24、82. Remove Duplicates from Sorted List II 去掉重复的数字（一个都不留） https://leetcode.com/problems/remove-duplicates-from-sorted-list-ii/#/description
没什么难点。遍历一次就行了
25、445. Add Two Numbers II 链表相加（与之前一体的区别是反过来了，而且不允许反转链表）
1、首先想到的应该是反转链表，然后求出答案，这样空间复杂度是O(n);
2、借助辅助空间，stack。
3、日常递归实现。
/**
* Definition for singly-linked list.
* public class ListNode {
* int val;
* ListNode next;
* ListNode(int x) { val = x; }
* }
*/
public class Solution {
public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
Stack<Integer> l1_stack = new Stack();
Stack<Integer> l2_stack = new Stack();
while (l1 != null){
l1_stack.add(l1.val);
l1 = l1.next;
}
while (l2 != null){
l2_stack.add(l2.val);
l2 = l2.next;
}
ListNode head = new ListNode(0), curr = null;
int flag = 0;
while (!l1_stack.isEmpty() || !l2_stack.isEmpty() || flag == 1){
int num = flag;
if (!l1_stack.isEmpty()){
num += l1_stack.pop();
}
if (!l2_stack.isEmpty()){
num += l2_stack.pop();
}
if (num > 9){
num -= 10;
flag = 1;
} else {
flag = 0;
}
ListNode node = new ListNode(num);
node.next = curr;
curr = node;
head.next = node;
}
return head.next;
}
}
/**
* Definition for singly-linked list.
* public class ListNode {
* int val;
* ListNode next;
* ListNode(int x) { val = x; }
* }
*/
public class Solution {
public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
int len1 = getLen(l1);
int len2 = getLen(l2);
ListNode head = new ListNode(1);
head.next = len1 > len2 ? helper(l1, l2, len1 - len2) : helper(l2, l1, len2 - len1);
if (head.next.val > 9){
head.next.val -= 10;
return head;
}
return head.next;
}
public ListNode helper(ListNode l1, ListNode l2, int offset){
if (l1 == null){
return null;
}
ListNode node = offset == 0 ? new ListNode(l1.val + l2.val) : new ListNode(l1.val);
ListNode next = offset == 0 ? helper(l1.next, l2.next, 0) : helper(l1.next, l2, offset - 1);
if (next != null && next.val > 9){
next.val -= 10;
node.val += 1;
}
node.next = next;
return node;
}
public int getLen(ListNode node){
int len = 0;
while (node != null){
node = node.next;
len++;
}
return len;
}
}
26、379. Design Phone Directory 设计电话字典。设计题。并不知道哪里用到了链表。 https://leetcode.com/problems/design-phone-directory/#/description
a、直接用数组就可以实现。
b、用队列 + set可以实现（更快一些，数组每次get需要遍历）。
这里用了数组实现。
public class PhoneDirectory {
/** Initialize your data structure here
@param maxNumbers - The maximum numbers that can be stored in the phone directory. */
private int[] phone;
public PhoneDirectory(int maxNumbers) {
phone = new int[maxNumbers];
for (int i = 0; i < maxNumbers; i++){
phone[i] = 1;
}
}
/** Provide a number which is not assigned to anyone.
@return - Return an available number. Return -1 if none is available. */
public int get() {
for (int i = 0; i < phone.length; i++){
if (phone[i] == 1){
phone[i] = 0;
return i;
}
}
return -1;
}
/** Check if a number is available or not. */
public boolean check(int number) {
if (number < 0 || number > phone.length){
return false;
}
return phone[number] == 1;
}
/** Recycle or release a number. */
public void release(int number) {
phone[number] = 1;
}
}
/**
* Your PhoneDirectory object will be instantiated and called as such:
* PhoneDirectory obj = new PhoneDirectory(maxNumbers);
* int param_1 = obj.get();
* boolean param_2 = obj.check(number);
* obj.release(number);
*/
27、369. Plus One Linked List 链表+1 https://leetcode.com/problems/plus-one-linked-list/#/description
1、递归。、
2、判断末尾有多少个连续的9即可。
/**
* Definition for singly-linked list.
* public class ListNode {
* int val;
* ListNode next;
* ListNode(int x) { val = x; }
* }
*/
public class Solution {
public ListNode plusOne(ListNode head) {
ListNode node = new ListNode(1);
node.next = head;
plusOneHelper(head);
if (node.next.val > 9){
node.next.val -= 10;
return node;
}
return node.next;
}
public void plusOneHelper(ListNode node){
if (node.next == null){
node.val += 1;
return ;
} else {
plusOneHelper(node.next);
if (node.next.val > 9){
node.next.val -= 10;
node.val += 1;
}
}
return ;
}
}