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链表总结

1. 增加表头元素
2. 倒数节点，使用快慢指针
3. 环形链表（快慢指针）
4. 合并有序链表，归并排序
5. LRU缓存

算法题

删除链表元素

删除链表中的节点

LeetCode237. 删除链表中的节点

复制后一个节点的值，删除后面的节点（1->5->3->4，删除5的话，先调整为1->3->3->4，再删除第二个3的节点）

class Solution {public void deleteNode(ListNode node) {node.val = node.next.val;node.next = node.next.next;}
}

删除链表的倒数第 N 个结点

LeetCode19. 删除链表的倒数第 N 个结点

快慢节点，使用虚拟节点，删除节点

当fast的next节点到了链表外，slow的next节点是第n个节点。找到slow的next节点，删除。

class Solution_LC19 {public ListNode removeNthFromEnd(ListNode head, int n) {ListNode dummy = new ListNode(-1);dummy.next = head;ListNode slow = dummy;ListNode fast = dummy;for (int i = 0; i < n; i++) {fast = fast.next;}while (fast.next != null) {fast = fast.next;slow = slow.next;}slow.next = slow.next.next;return dummy.next;}}

删除排序链表中的重复元素

LeetCode83 删除排序链表中的重复元素

和当前节点比较值，相同则删掉，不同则下一个节点

class Solution_LC83 {public ListNode deleteDuplicates(ListNode head) {ListNode cur = head;while (cur != null) {int val = cur.val;if (cur.next != null && cur.next.val == val) {cur.next = cur.next.next;} else {cur = cur.next;}}return head;}
}

删除排序链表中的重复元素 II（**）

LeetCode82. 删除排序链表中的重复元素 II

定义两个节点。cur节点是用来比较的节点，pre节点是用来删除的。找到cur节点，该节点和next节点不一致，pre.next=cur，等于是删除了pre和cur之间的元素。

class Solution {public ListNode deleteDuplicates(ListNode head) {ListNode dummy = new ListNode(-1);dummy.next = head;ListNode pre = dummy;ListNode cur = head;while (cur != null && cur.next != null) {int x = cur.val;if (cur.next.val == x) {while (cur != null && cur.val == x) {cur = cur.next;}pre.next = cur;} else {cur = cur.next;pre = pre.next;}}return dummy.next;}
}

旋转链表

反转链表

LeetCode206. 反转链表

头插法。pre和cur不断向后移动，直到cur为空，pre为最后一个节点（遍历顺序的最后一个）。

class Solution {public ListNode reverseList(ListNode head) {ListNode pre = null;ListNode cur = head;while (cur != null) {ListNode next = cur.next;cur.next = pre;pre = cur;cur = next;}return pre;}
}

K 个一组翻转链表

LeetCode25. K 个一组翻转链表

• 获取k个节点一组的链表

• 翻转链表

• pre的后面一个节点是start，end的最后一个节点是next。

class Solution {public ListNode reverseKGroup(ListNode head, int k) {ListNode dummy = new ListNode(-1);dummy.next = head;ListNode pre = dummy;ListNode end = dummy;while (end.next != null) {for (int i = 0; i < k&&end!=null; i++) {end = end.next;}if (end == null) {break;}ListNode next = end.next;ListNode start = pre.next;end.next = null;pre.next = reverse(start);start.next = next;pre = start;end = start;}return dummy.next;}private ListNode reverse(ListNode head) {ListNode pre = null;ListNode cur = head;while (cur != null) {ListNode next = cur.next;cur.next = pre;pre = cur;cur = next;}return pre;}
}

LeetCode61. 旋转链表

LeetCode61. 旋转链表.

• 获取链表的尾结点

• 尾结点连接头节点

• 找到切割点（切割点的前一个节点）

• 切割。获取next节点，将当前节点的next置为空，切断。

class Solution {public ListNode rotateRight(ListNode head, int k) {if (k == 0 || head == null || head.next == null) {return head;}ListNode cur = head;int n = 1;while (cur.next != null) {cur = cur.next;n++;}int add = n - k % n;if (add == n) {return head;}cur.next = head;while (add > 0) {cur = cur.next;add--;}ListNode next = cur.next;cur.next = null;return next;}
}

交换链表节点

LeetCode24. 两两交换链表中的节点（⭐️高频）

LeetCode24. 两两交换链表中的节点

定义虚拟头结点

获取可以交换的节点，进行节点操作

将node1节点置为前置结点，进行下一轮操作

class Solution {public ListNode swapPairs(ListNode head) {ListNode dummy = new ListNode(-1, head);ListNode cur = dummy;while (cur.next != null && cur.next.next != null) {ListNode node1 = cur.next;ListNode node2 = node1.next;ListNode next = node2.next;cur.next = node2;node2.next = node1;node1.next = next;cur = node1;}return dummy.next;}
}

环形/相交/回文链表

LeetCode141. 环形链表（腾讯）

LeetCode141. 环形链表（腾讯）

使用快慢指针，如果快指针最后到达慢指针，则存在环。

public class Solution {public boolean hasCycle(ListNode head) {ListNode fast = head;ListNode slow = head;while (fast != null && fast.next != null) {fast = fast.next.next;slow = slow.next;if (fast == slow) {return true;}}return false;}
}

LeetCode142. 环形链表II

LeetCode142. 环形链表II.

快慢指针

a+b+n(b+c)=2(a+b) --> a=(n-1)(b+c)+c

public class Solution {public ListNode detectCycle(ListNode head) {ListNode fast = head;ListNode slow = head;while (fast != null && fast.next != null) {fast = fast.next.next;slow = slow.next;if (slow == fast) {ListNode node1 = head;ListNode node2 = fast;while (node1 != node2) {node1 = node1.next;node2 = node2.next;}return node1;}}return null;}
}

LeetCode160.相交链表

160. 相交链表

要进行临界值判断

相交的链表后面一段是公共的，a+b+c=c+b+a。

public class Solution {public ListNode getIntersectionNode(ListNode headA, ListNode headB) {if (headA == null || headB == null) {return null;}ListNode p1 = headA;ListNode p2 = headB;while (p1 != p2) {p1 = p1 == null ? headB : p1.next;p2 = p2 == null ? headA : p2.next;}return p1;}
}

LeetCode234. 回文链表

234. 回文链表

寻找中间节点，并反转前面列表

pre是反转链表的头结点，slow是后面链表的头结点。比较节点的值，判断是否回文

class Solution {public boolean isPalindrome(ListNode head) {//1-2-3-2-1ListNode fast = head;ListNode slow = head;ListNode pre = null;while (fast != null && fast.next != null) {fast = fast.next.next;ListNode next = slow.next;slow.next = pre;pre = slow;slow = next;}if (fast != null) {slow = slow.next;}while (pre != null && slow != null) {if (slow.val != pre.val) {return false;} else {slow = slow.next;pre = pre.next;}}return true;}
}

链表合并

LeetCode2: 两数相加

LeetCode2: 两数相加

对应数位的值相加，计算当前节点以及向上的值。

class Solution {public ListNode addTwoNumbers(ListNode l1, ListNode l2) {ListNode dummy = new ListNode(-1);ListNode cur = dummy;int carry = 0;while (l1 != null || l2 != null || carry != 0) {int sum = (l1 == null ? 0 : l1.val) + (l2 == null ? 0 : l2.val) + carry;carry = sum / 10;ListNode tmp = new ListNode(sum % 10);cur.next = tmp;cur = tmp;l1 = l1 == null ? null : l1.next;l2 = l2 == null ? null : l2.next;}return dummy.next;}
}

LeetCode445: 两数相加II

LeetCode445: 两数相加II

使用堆栈，用于顺序相反获取值

链表的拼接和上一题不同。上一题是不断往链表后面添加元素；这一题是不断往前面添加元素。

class Solution {public ListNode addTwoNumbers(ListNode l1, ListNode l2) {Stack<Integer> stack1 = new Stack<>();Stack<Integer> stack2 = new Stack<>();while (l1 != null) {stack1.push(l1.val);l1 = l1.next;}while (l2 != null) {stack2.push(l2.val);l2 = l2.next;}int carry = 0;ListNode cur = null;while (!stack1.isEmpty() || !stack2.isEmpty() || carry != 0) {int sum = (stack1.isEmpty() ? 0 : stack1.pop()) + (stack2.isEmpty() ? 0 : stack2.pop()) + carry;carry = sum / 10;ListNode tmp = new ListNode(sum % 10);tmp.next = cur;cur = tmp;}return cur;}
}

LeetCode21: 合并两个有序链表

21. 合并两个有序链表

挨个遍历比较大小，是最容易想到的方案

使用递归。当l1.val < l2.val，l1.next = mergeTwoLists(l1.next, l2);

class Solution {public ListNode mergeTwoLists(ListNode list1, ListNode list2) {ListNode dummy = new ListNode(-1);ListNode cur = dummy;while (list1 != null && list2 != null) {if (list1.val < list2.val) {cur.next = new ListNode(list1.val);cur = cur.next;list1 = list1.next;} else {cur.next = new ListNode(list2.val);cur = cur.next;list2 = list2.next;}}if (list1 != null) {cur.next = list1;} else {cur.next = list2;}return dummy.next;}
}

LeetCode23: 合并K个排序链表

23. 合并 K 个升序链表

挨个遍历处理

class Solution {public ListNode mergeKLists(ListNode[] lists) {ListNode ans = null;for (int i = 0; i < lists.length; i++) {ans =mergeTwoLists(ans, lists[i]);}return ans;}public ListNode mergeTwoLists(ListNode list1, ListNode list2) {if (list1 == null || list2 == null) {return list1 == null ? list2 : list1;}ListNode dummy = new ListNode(-1);ListNode cur = dummy;while (list1 != null && list2 != null) {if (list1.val < list2.val) {cur.next = new ListNode(list1.val);cur = cur.next;list1 = list1.next;} else {cur.next = new ListNode(list2.val);cur = cur.next;list2 = list2.next;}}if (list1 != null) {cur.next = list1;} else {cur.next = list2;}return dummy.next;}
}

分治合并，将链表数组拆分成2段，处理好后合并。

class Solution {public ListNode mergeKLists(ListNode[] lists) {return merge(lists, 0, lists.length - 1);}private ListNode merge(ListNode[] lists, int l, int r) {if (l == r) {return lists[l];}if (l > r) {return null;}int mid = (l + r) / 2;return mergeTwoLists(merge(lists, l, mid), merge(lists, mid + 1, r));}public ListNode mergeTwoLists(ListNode list1, ListNode list2) {if (list1 == null || list2 == null) {return list1 == null ? list2 : list1;}ListNode dummy = new ListNode(-1);ListNode cur = dummy;while (list1 != null && list2 != null) {if (list1.val < list2.val) {cur.next = new ListNode(list1.val);cur = cur.next;list1 = list1.next;} else {cur.next = new ListNode(list2.val);cur = cur.next;list2 = list2.next;}}if (list1 != null) {cur.next = list1;} else {cur.next = list2;}return dummy.next;}
}

使用优先队列

class Solution {public ListNode mergeKLists(ListNode[] lists) {//优先队列默认是小顶堆，最小的元素放在队头，即a-bPriorityQueue<ListNode> priorityQueue = new PriorityQueue<ListNode>((a, b) -> {return a.val - b.val;});for (int i = 0; i < lists.length; i++) {if(lists[i]!=null){
priorityQueue.add(lists[i]);}}ListNode dummy = new ListNode(-1);ListNode cur = dummy;while (!priorityQueue.isEmpty()) {ListNode listNode = priorityQueue.poll();ListNode next = listNode.next;cur.next = listNode;cur = listNode;if (next != null) {priorityQueue.add(next);}}return dummy.next;}
}

LeetCode148: 排序链表

148. 排序链表

使用优先队列，最简单。

使用归并排序。做链表拆分。

可以和回文链表做比较，必须熟悉掌握两个链表合并的逻辑。

class Solution {public ListNode sortList(ListNode head) {if (head == null || head.next == null) {return head;}ListNode fast = head.next;ListNode slow = head;while (fast != null && fast.next != null) {fast = fast.next.next;slow = slow.next;}ListNode next = slow.next;slow.next = null;ListNode listNode1 = sortList(head);ListNode listNode2 = sortList(next);ListNode dummy = new ListNode(-1);ListNode cur =dummy;while (listNode1 != null && listNode2 != null) {if (listNode1.val < listNode2.val) {cur.next = listNode1;cur = cur.next;listNode1 = listNode1.next;} else {cur.next = listNode2;cur = cur.next;listNode2 = listNode2.next;}}cur.next = listNode1 == null ? listNode2 : listNode1;return dummy.next;}
}

LRU缓存

LeetCode146. LRU 缓存

146. LRU 缓存

使用LinkedHashMap，设置accessOrder为true，最近访问的元素会排在最后。而removeEldestEntry当条件满足的时候会移除最老的元素。

class LRUCache extends LinkedHashMap<Integer, Integer> {private int capacity;public LRUCache(int capacity) {super(capacity, 0.75f, true);this.capacity = capacity;}public int get(int key) {return super.getOrDefault(key, -1);}public void put(int key, int value) {super.put(key, value);}protected boolean removeEldestEntry(Map.Entry<Integer, Integer> entry) {return size() > this.capacity;}
}

使用Hash表和双向链表

双向链表记录访问顺序，Hash表获取元素

获取元素，将元素放在最前（移除当前元素在双向链表中的原位置，放在前面）。

存放元素，如果元素已存在，进行更新值，且将元素放在最前；如果元素不存在，添加元素要判断边界，超过边界要移除最早访问的元素（从链表和Hash表中移除）。

class LRUCache extends LinkedHashMap<Integer, Integer> {public class DLinkedNode {int key;int val;DLinkedNode prev;DLinkedNode next;public DLinkedNode() {}public DLinkedNode(int key, int val) {this.key = key;this.val = val;}}private Map<Integer, DLinkedNode> map = new HashMap<>();private int size;private int capacity;private DLinkedNode head, tail;public LRUCache(int capacity) {this.capacity = capacity;size = 0;head = new DLinkedNode();tail = new DLinkedNode();head.next = tail;tail.prev = head;}public int get(int key) {DLinkedNode dLinkedNode = map.get(key);if (dLinkedNode == null) {return -1;} else {moveToHead(dLinkedNode);return dLinkedNode.val;}}private void moveToHead(DLinkedNode node) {removeNode(node);addToHead(node);}private void addToHead(DLinkedNode node) {DLinkedNode next = head.next;head.next = node;node.prev = head;node.next = next;next.prev = node;}private void removeNode(DLinkedNode node) {DLinkedNode prev = node.prev;DLinkedNode next = node.next;prev.next = next;next.prev = prev;}private DLinkedNode removeTail() {DLinkedNode prev = tail.prev;removeNode(prev);return prev;}public void put(int key, int value) {DLinkedNode node = map.get(key);if (node == null) {DLinkedNode newNode = new DLinkedNode(key, value);map.put(key, newNode);addToHead(newNode);size++;if (size > capacity) {DLinkedNode tail = removeTail();map.remove(tail.key);size--;}} else {node.val = value;moveToHead(node);}}
}