链表

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https://oj.leetcode.com/tag/linked-list/

#1 分与合

分的办法：#1 奇偶分(不容易错) #2 快慢中分 #3根据长度中分复杂度与快慢中分一模一样

interface Divider {    public ListNode divide(ListNode head);}

// 奇偶中分class EvenOddDivider implements Divider {    public ListNode divide(ListNode head) {        ListNode evnHead = head;        ListNode oddHead = head.next;        ListNode evnTail = evnHead;        ListNode oddTail = oddHead;        ListNode tail = oddHead.next;        int count = 2;        while (tail != null) {            if (count++ % 2 == 0) {                evnTail.next = tail;                evnTail = tail;            } else {                oddTail.next = tail;                oddTail = tail;            }            tail = tail.next;        }        evnTail.next = null;        oddTail.next = null;        return oddHead;    }}

// 长度中分class LengthDivider implements Divider {    // 小心没有分成两份 造成死循环    public ListNode divide(ListNode head) {        int len = length(head);        int mid = len / 2;        for (int i = 0; i < mid - 1; i++) { //如果i=0 当len==2时 没有分成两段 造成死循环            head = head.next;        }        ListNode result = head.next;        head.next = null;        return result;    }        private int length(ListNode head) {        int length = 0;        while(head != null) {            length++;            head = head.next;        }        return length;    }}

// 快慢中分class SlowFastDivider implements Divider {    // 小心没有分成两份 造成死循环    public ListNode divide(ListNode head) {        ListNode slow = head;        ListNode fast = head.next; // 如果不降fast指向head.next，那么没有分成两段 造成死循环        while(fast != null && fast.next != null) {            fast = fast.next.next;            slow = slow.next;        }        ListNode result = slow.next;        slow.next = null;        return result;    }}class SlowFastDivider2 implements Divider {    public ListNode divide(ListNode head) {        ListNode slow = head;        ListNode fast = head;        while(fast != null) {            fast = fast.next;            if (fast != null) {                fast = fast.next;                if (fast != null) {                    slow = slow.next;                }            }        }        ListNode result = slow.next;        slow.next = null;        return result;    }}

合的办法：#1 有序合 #2拉锁合

https://oj.leetcode.com/problems/sort-list/ (随意分，递归，有序合)

A4：https://oj.leetcode.com/problems/merge-two-sorted-lists/ （有序合）

C23: https://oj.leetcode.com/problems/convert-sorted-list-to-binary-search-tree/ （快慢中分）

E2：https://oj.leetcode.com/problems/swap-nodes-in-pairs/ （奇偶分，拉锁合）

E7：https://oj.leetcode.com/problems/reorder-list/ （快慢中分，逆转，拉链合）

E12：https://oj.leetcode.com/problems/copy-list-with-random-pointer/ （clone到一起，连接random，然后分开。不需要使用map）

#2 逆转

public ListNode reverse(ListNode head) {    ListNode fake = new ListNode(0);    while(head != null) {        ListNode next = head.next;        head.next = fake.next;        fake.next = head;        head = next;    }    return fake.next;}

// 返回第k个节点 找不到返回null 第一个是headprivate ListNode getKNode(ListNode head, int k) {    if(k <= 0) return null;    for(int i = 0; i < k - 1 && head != null; i++) {        head = head.next;    }    return head;}

E3：https://oj.leetcode.com/problems/reverse-nodes-in-k-group/ （分段逆转用了getK）

E5：https://oj.leetcode.com/problems/reverse-linked-list-ii/ （分三段逆转中段用了getK）

E7：https://oj.leetcode.com/problems/reorder-list/ （快慢中分，逆转，拉链合）

#3 快慢指针

找环，找中点，中分链表

C23: https://oj.leetcode.com/problems/convert-sorted-list-to-binary-search-tree/ （快慢分）

E4：https://oj.leetcode.com/problems/remove-nth-node-from-end-of-list/

E6：https://oj.leetcode.com/problems/rotate-list/

E9：https://oj.leetcode.com/problems/linked-list-cycle/ （重）

https://oj.leetcode.com/problems/linked-list-cycle-ii/ （可以转成E10 比较省事）

E10：https://oj.leetcode.com/problems/intersection-of-two-linked-lists/

#4 去重

E1：https://oj.leetcode.com/problems/remove-duplicates-from-sorted-list/ (同一链表，cur跟next比较，要不要next)

https://oj.leetcode.com/problems/remove-duplicates-from-sorted-list-ii/ (创建新链表。cur跟pre没冲突 cur跟next没冲突 cur往新链表放)

#5 其它

E11：https://oj.leetcode.com/problems/add-two-numbers/ （递归）

#6 排序相关

https://oj.leetcode.com/problems/sort-list/

A2：https://oj.leetcode.com/problems/partition-list/

A4：https://oj.leetcode.com/problems/merge-two-sorted-lists/

A4-B：https://oj.leetcode.com/problems/merge-k-sorted-lists/

E8：https://oj.leetcode.com/problems/insertion-sort-list/

# 7二叉树相关

C23: https://oj.leetcode.com/problems/convert-sorted-list-to-binary-search-tree/ （快慢中分）（重）

C24: https://oj.leetcode.com/problems/flatten-binary-tree-to-linked-list/ （后序先右后左）

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