算法练习（5）：牛客在线编程01 链表

package jz.bm;

import jz.ListNode;

import java.util.ArrayList;
import java.util.Collections;
import java.util.HashMap;
import java.util.HashSet;

public class bm1 {
    /**
     * BM1 反转链表
     */
    public ListNode ReverseList(ListNode head) {
        if (head == null) {
            return null;
        }
        ListNode pre = head;
        ListNode cur = head.next;
        pre.next = null;
        while (cur != null) {
            ListNode next = cur.next;
            cur.next = pre;
            pre = cur;
            cur = next;
        }
        return pre;
    }

    /**
     * BM2 链表内指定区间反转
     */
    public ListNode reverseBetween (ListNode head, int m, int n) {
        ListNode temp = new ListNode(-1);
        temp.next = head;

        int index = 1;
        ListNode mPre = temp;
        ListNode mNode = head;
        while (index < m) {
            mPre = mNode;
            mNode = mNode.next;
            index++;
        }

        ListNode nPre = null;
        ListNode nNode = mNode;
        while (index < n) {
            ListNode next = nNode.next;
            nNode.next = nPre;
            nPre = nNode;
            nNode = next;
            index++;
        }

        mPre.next = nPre;
        mNode.next = nNode;

        return temp.next;
    }

    /**
     * BM3 链表中的节点每k个一组翻转
     */
    public ListNode reverseKGroup (ListNode head, int k) {
        if(head == null) {
            return null;
        }

        int n = 1;
        ListNode cur = head;
        while (n <= k) {
            if(cur == null) {
                return head;
            }
            cur = cur.next;
            n++;
        }

        ListNode pre = null;
        cur = head;
        n = 1;
        while (n <= k) {
            ListNode next = cur.next;
            cur.next = pre;
            pre = cur;
            cur = next;
            n++;
        }
        head.next = reverseKGroup(cur, k);
        return pre;
    }

    /**
     * BM4 合并两个排序的链表
     */
    public ListNode Merge(ListNode list1,ListNode list2) {
        if (list1 == null) {
            return list2;
        }
        if (list2 == null) {
            return list1;
        }
        ListNode temp = new ListNode(-1);
        ListNode cur = temp;
        while (list1 != null && list2 != null) {
            if (list1.val < list2.val) {
                cur.next = list1;
                list1 = list1.next;
            } else {
                cur.next = list2;
                list2 = list2.next;
            }
            cur = cur.next;
        }
        while (list1 != null) {
            cur.next = list1;
            list1 = list1.next;
            cur = cur.next;
        }
        while (list2 != null) {
            cur.next = list2;
            list2 = list2.next;
            cur = cur.next;
        }
        return temp.next;
    }

    /**
     * BM5 合并k个已排序的链表
     */
    public ListNode mergeKLists (ArrayList<ListNode> lists) {
        return divideMergeKLists(lists, 0, lists.size() - 1);
    }

    public ListNode divideMergeKLists (ArrayList<ListNode> lists, int left, int right) {
        if (left > right) {
            return null;
        } else if (left == right) { //只有一个链表，直接返回
            return lists.get(left);
        }
        int mid = (left + right) / 2;
        return Merge(divideMergeKLists(lists, left, mid), divideMergeKLists(lists, mid + 1, right));
    }

    /**
     * BM6 判断链表中是否有环
     */
    public boolean hasCycle(ListNode head) {
        ListNode slow = head;
        ListNode fast = head;
        while (fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
            if (fast == slow) {
                return true;
            }
        }
        return true;
    }

    /**
     * BM7 链表中环的入口结点
     */
    public ListNode EntryNodeOfLoop(ListNode pHead) {
        //快慢指针
        ListNode slow = pHead;
        ListNode fast = pHead;
        while(fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
            //有环，找到相遇点
            if (slow == fast) {
                //两个指针同时走，相遇点就是环起始点
                fast = pHead;
                while(fast != null) {
                    if (slow == fast) {
                        return slow;
                    }
                    fast = fast.next;
                    slow = slow.next;
                }
            }
        }
        return null;
    }

    /**
     * BM8 链表中倒数最后k个结点
     */
    public ListNode FindKthToTail (ListNode pHead, int k) {
        //双指针
        ListNode slow = pHead;
        ListNode fast = pHead;
        for (int i = 0; i < k; i++) {
            if (fast != null) {
                fast = fast.next;
            } else {
                return null;
            }
        }
        while (fast != null) {
            slow = slow.next;
            fast = fast.next;
        }
        return slow;
    }

    /**
     * BM9 删除链表的倒数第n个节点
     */
    public ListNode removeNthFromEnd (ListNode head, int n) {
        ListNode temp = new ListNode(-1);
        temp.next = head;
        int len = 0;
        ListNode cur = head;
        while (cur != null) {
            len++;
            cur = cur.next;
        }

        ListNode pre = temp;
        cur = head;
        for (int i = 0; i < len; i++) {
            if(i == len - n) {
                pre.next = cur.next;
                break;
            } else {
                pre = cur;
                cur = cur.next;
            }
        }
        return temp.next;
    }

    /**
     * BM10 两个链表的第一个公共结点
     */
    public ListNode FindFirstCommonNode(ListNode pHead1, ListNode pHead2) {
        if (pHead1 == null) {
            return null;
        }
        if (pHead2 == null) {
            return null;
        }
        HashSet<ListNode> set = new HashSet<>();
        ListNode cur = pHead1;
        while (cur != null) {
            set.add(cur);
            cur = cur.next;
        }
        cur = pHead2;
        while (cur != null) {
            if (set.contains(cur)) {
                return cur;
            }
            cur = cur.next;
        }
        return null;
    }

    /**
     * BM11 链表相加(二)
     */
    public ListNode addInList (ListNode head1, ListNode head2) {
        head1 = ReverseList(head1);
        head2 = ReverseList(head2);
        int temp = 0;
        ListNode head = new ListNode(-1);
        ListNode cur = head;
        while (head1 != null || head2 != null || temp != 0) {
            int v1 = head1 == null ? 0 : head1.val;
            int v2 = head2 == null ? 0 : head2.val;
            cur.next = new ListNode((v1 + v2 + temp) % 10);
            temp = (v1 + v2 + temp) / 10;
            cur = cur.next;
            head1 = head1 == null ? null : head1.next;
            head2 = head2 == null ? null : head2.next;
        }
        return ReverseList(head.next);
    }

    /**
     * BM12 单链表的排序
     */
    public ListNode sortInList (ListNode head) {
        ArrayList<Integer> list = new ArrayList<>();
        ListNode cur = head;
        while (cur != null) {
            list.add(cur.val);
            cur = cur.next;
        }
        Collections.sort(list);
        cur = head;
        for (Integer integer : list) {
            cur.val = integer;
            cur = cur.next;
        }
        return head;
    }

    /**
     * BM13 判断一个链表是否为回文结构
     */
    public boolean isPail (ListNode head) {
        ArrayList<Integer> list = new ArrayList<>();
        while (head != null) {
            list.add(head.val);
            head = head.next;
        }
        int i = 0, j = list.size() - 1;
        while (i < j) {
            if (!list.get(i).equals(list.get(j))) {
                return false;
            }
            i++;
            j--;
        }
        return true;
    }

    /**
     * BM15 删除有序链表中重复的元素-I
     */
    public ListNode deleteDuplicates (ListNode head) {
        ListNode cur = head;
        while (cur != null && cur.next != null) {
            if (cur.val == cur.next.val) {
                cur.next = cur.next.next;
            } else {
                cur = cur.next;
            }
        }
        return head;
    }

    /**
     * BM16 删除有序链表中重复的元素-II
     */
    public ListNode deleteDuplicates2 (ListNode head) {
        ListNode fake = new ListNode(1001);
        fake.next = head;

        ListNode cur = fake;
        while(cur != null && cur.next != null && cur.next.next != null) {
            //发现有重复节点
            if (cur.next.val == cur.next.next.val) {
                int val = cur.next.val;
                //找到最后一个重复节点
                ListNode tmp = cur.next.next;
                while (tmp.next != null && val == tmp.next.val) {
                    tmp = tmp.next;
                }
                //重新连接
                cur.next = tmp.next;
                //当前节点不变
            } else {
                cur = cur.next;
            }
        }
        return fake.next;
    }
}