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本系列为笔者的 Leetcode 刷题记录，顺序为 Hot 100 题官方顺序，根据标签命名，记录笔者总结的做题思路，附部分代码解释和疑问解答，01~07为C++语言，08及以后为Java语言。

01 合并两个有序链表

/*** Definition for singly-linked list.* public class ListNode {*     int val;*     ListNode next;*     ListNode() {}*     ListNode(int val) { this.val = val; }*     ListNode(int val, ListNode next) { this.val = val; this.next = next; }* }*/
class Solution {public ListNode mergeTwoLists(ListNode list1, ListNode list2) {}
}

方法一：递归

class Solution {public ListNode mergeTwoLists(ListNode list1, ListNode list2) {if(list1 == null){return list2;}else if(list2 == null){return list1;}else if(list1.val < list2.val){list1.next = mergeTwoLists(list1.next, list2);return list1;}else{list2.next = mergeTwoLists(list1, list2.next);return list2;}}
}

方法二：遍历

class Solution {public ListNode mergeTwoLists(ListNode list1, ListNode list2) {ListNode list3 = new ListNode(-1);ListNode flag = list3; //论标记的重要性while(list1 != null && list2 != null){if(list1.val < list2.val){list3.next = list1;list1 = list1.next;}else{list3.next = list2;list2 = list2.next;}list3 = list3.next;}if(list1 != null){list3.next = list1;}else{list3.next = list2;}return flag.next; //返回头结点指针}
}

02 两数相加

方法一：设新链表、进位值，不断创建新节点

/*** Definition for singly-linked list.* public class ListNode {*     int val;*     ListNode next;*     ListNode() {}*     ListNode(int val) { this.val = val; }*     ListNode(int val, ListNode next) { this.val = val; this.next = next; }* }*/
class Solution {public ListNode addTwoNumbers(ListNode l1, ListNode l2) {ListNode start = null, end = null;int flag = 0;while(l1 != null || l2 != null){//1. 计算单位加和int n1 = l1 != null ? l1.val : 0;int n2 = l2 != null ? l2.val : 0;int sum = n1 + n2 + flag;//2. 添加新链表结点，计算进位if(start == null){start = end = new ListNode(sum % 10);}else{end.next = new ListNode(sum % 10);end = end.next;}flag = sum / 10;//3. 移动结点if(l1 != null){l1 = l1.next;} if(l2 != null){l2 = l2.next;} }//4. 特殊情况：高位进一if(flag > 0){end.next = new ListNode(flag);}return start;}
}

03 删除链表的倒数第N个结点

/*** Definition for singly-linked list.* public class ListNode {*     int val;*     ListNode next;*     ListNode() {}*     ListNode(int val) { this.val = val; }*     ListNode(int val, ListNode next) { this.val = val; this.next = next; }* }*/
class Solution {public ListNode removeNthFromEnd(ListNode head, int n) {ListNode dummy = new ListNode(0, head);ListNode first = head;ListNode second = dummy; //可移动结点//1. first移动n位for(int i=0; i<n; ++i){first = first.next;}//2. first、second同时移动，first=null为止while(first != null){first = first.next;second = second.next;}//3. 删除结点，释放空间second.next = second.next.next;return dummy.next;}
}

① dummy.next不是head吗，那么ListNode ans = dummy.next; return ans;存在的意义是什么？

如果 n 等于链表的长度，即需要删除头节点时，单独使用 head 会导致很复杂的边界条件处理，在这种情况下，dummy 提供一个可靠的起始节点，方便统一处理删除节点的逻辑。

② 为什么要新建一个链表ListNode dummy = new ListNode(0, head);，而不是在原有链表上操作？

dummy节点简化了边界条件的处理，所有节点，无论是否是头节点，都可以以一致的方式处理，最终可以直接返回dummy.next作为结果，这样不会影响原始链表的头节点指向。

04 两两交换链表中的节点

/*** Definition for singly-linked list.* public class ListNode {*     int val;*     ListNode next;*     ListNode() {}*     ListNode(int val) { this.val = val; }*     ListNode(int val, ListNode next) { this.val = val; this.next = next; }* }*/
class Solution {public ListNode swapPairs(ListNode head) {ListNode dummy = new ListNode(0, head);ListNode flag = dummy; //可移动结点while(flag.next != null && flag.next.next != null){//1. 顺次两个节点ListNode a = flag.next;ListNode b = flag.next.next;//2. 交换flag.next = b;a.next = b.next;b.next = a;flag = a;}return dummy.next; //万金油}
}

05 K个一组翻转链表

/*** Definition for singly-linked list.* public class ListNode {*     int val;*     ListNode next;*     ListNode() {}*     ListNode(int val) { this.val = val; }*     ListNode(int val, ListNode next) { this.val = val; this.next = next; }* }*/
class Solution {//需要pre,head,tail,nex四个指针public ListNode reverseKGroup(ListNode head, int k) {//1.创建dummy链表,返回dummy.nextListNode dummy = new ListNode(0, head);ListNode pre = dummy;//2.寻找(head,tail),调用myReverse方法while(head != null){ListNode tail = pre;for(int i=0; i<k; ++i){tail = tail.next;if(tail == null){return dummy.next;}}ListNode nex = tail.next;ListNode[] reverse = myReverse(head, tail);head = reverse[0];tail = reverse[1];//3.连接断裂链表pre.next = head;tail.next = nex;pre = tail;head = nex;}return dummy.next;}//需要a,b,c三个指针public ListNode[] myReverse(ListNode start, ListNode end){//1.寻找a,b,c//2.核心操作:a->c//3.临界条件:end==cListNode a = start;ListNode c = end.next;while(end != c){ListNode b = a.next;a.next = c;c = a;a = b;}return new ListNode[]{end, start};}
}

06 随机链表的复制

失败代码

/*
// Definition for a Node.
class Node {int val;Node next;Node random;public Node(int val) {this.val = val;this.next = null;this.random = null;}
}
*/class Solution {public Node copyRandomList(Node head) {//1.创建dummy链表,返回dummy.nextNode dummy = new Node(0);dummy.next = head;//2.第一次遍历,复制结点值和next指针Node end = dummy;Node start = head;while(start != null){end.next = new Node(start.val); //复制结点值end = end.next;end.next = start.next; //复制next指针start = start.next;}//3.第二次遍历,复制random指针Node end2 = dummy;Node start2 = head;while(start2 != null){end2 = end2.next;end2.random = start2.random; //复制random指针start2 = start2.next;}return dummy.next;}
}

问题：当前复制的 random 指针仍然指向原链表中的节点，而不是新复制链表中的节点。

方法一：递归

/*
// Definition for a Node.
class Node {int val;Node next;Node random;public Node(int val) {this.val = val;this.next = null;this.random = null;}
}
*/class Solution {//1.创建映射Map<Node, Node> cachedNode = new HashMap<Node, Node>();public Node copyRandomList(Node head) {if(head == null){return null;}//2.创建新节点，拷贝valif(!cachedNode.containsKey(head)){Node headNew = new Node(head.val);cachedNode.put(head, headNew);//3.递归，拷贝next,randomheadNew.next = copyRandomList(head.next);headNew.random = copyRandomList(head.random);}return cachedNode.get(head);}
}

方法二：结点拆分

/*
// Definition for a Node.
class Node {int val;Node next;Node random;public Node(int val) {this.val = val;this.next = null;this.random = null;}
}
*/class Solution {public Node copyRandomList(Node head) {if(head == null){return null;}//1. 一次遍历：节点插入，拷贝valfor(Node node = head; node != null; node = node.next.next){Node nodeNew = new Node(node.val);nodeNew.next = node.next;node.next = nodeNew;}//2. 二次遍历：拷贝randomfor(Node node = head; node != null; node = node.next.next){Node nodeNew = node.next;nodeNew.random = (node.random != null) ? node.random.next : null;}//3. 三次遍历：拷贝nextNode headNew = head.next;for(Node node = head; node != null; node = node.next){Node nodeNew = node.next;node.next = node.next.next;nodeNew.next = (node.next != null) ? nodeNew.next.next : null;}return headNew;}
}

07 排序链表

方法一：快慢指针归并排序 + 合并有序链表

/*** Definition for singly-linked list.* public class ListNode {*     int val;*     ListNode next;*     ListNode() {}*     ListNode(int val) { this.val = val; }*     ListNode(int val, ListNode next) { this.val = val; this.next = next; }* }*/
class Solution {public ListNode sortList(ListNode head) {return sortList(head, null); //没说区间，默认右端点为null }//1.伪二分（快慢指针）public ListNode sortList(ListNode head, ListNode tail){// if(head == null || head.next == tail){ //特殊情况判断//     return head;// }if (head == null) {return head;}if (head.next == tail) {head.next = null;return head;}ListNode fast = head;ListNode slow = head;while(fast != tail){fast = fast.next;slow = slow.next;if(fast != tail){fast = fast.next;}}//2.归并排序(head,tail)ListNode mid = slow;ListNode list1 = sortList(head, mid);ListNode list2 = sortList(mid, tail);ListNode sorted = mergeList(list1, list2);return sorted;}//3.合并两个有序链表public ListNode mergeList(ListNode l1, ListNode l2){ListNode l3 = new ListNode(0);ListNode flag = l3; //return flag.next;while(l1 != null && l2 != null){if(l1.val < l2.val){l3.next = l1;l1 = l1.next;}else{l3.next = l2;l2 = l2.next;}l3 = l3.next;}if(l1 != null){l3.next = l1;}if(l2 != null){l3.next = l2;}return flag.next;}
}

方法二：倍增长度 + 合并两个有序链表

/*** Definition for singly-linked list.* public class ListNode {*     int val;*     ListNode next;*     ListNode() {}*     ListNode(int val) { this.val = val; }*     ListNode(int val, ListNode next) { this.val = val; this.next = next; }* }*/
class Solution {public ListNode sortList(ListNode head) {if(head == null){ //特殊情况判断return head;}//1.获取链表长度lengthint length = 0;ListNode node = head;while(node != null){length++;node = node.next;}//2.倍增长度subLength//a.四指针 pre/head1/head2/nex//b.两断链 //c.一接骨ListNode dummy = new ListNode(0, head);for(int subLength = 1; subLength < length; subLength <<= 1){ListNode pre = dummy;ListNode curr = dummy.next;while(curr != null){ListNode head1 = curr;for(int i=1; i<length && curr.next != null; i++){curr = curr.next;}ListNode head2 = curr;curr.next = null;curr = head2;for(int i=1; i<length && curr.next != null && curr != null; i++){curr = curr.next;}ListNode nex = null;if(curr.next != null){nex = curr.next;curr.next = null;}ListNode merged = mergeList(head1, head2);pre.next = merged;pre = curr;curr = nex;}}//3.合并方法return dummy.next;}//合并两个有序链表public ListNode mergeList(ListNode l1, ListNode l2){ListNode l3 = new ListNode(0);ListNode flag = l3; //return flag.next;while(l1 != null && l2 != null){if(l1.val < l2.val){l3.next = l1;l1 = l1.next;}else{l3.next = l2;l2 = l2.next;}l3 = l3.next;}if(l1 != null){l3.next = l1;}if(l2 != null){l3.next = l2;}return flag.next;}
}

.next;
}

//合并两个有序链表
public ListNode mergeList(ListNode l1, ListNode l2){ListNode l3 = new ListNode(0);ListNode flag = l3; //return flag.next;while(l1 != null && l2 != null){if(l1.val < l2.val){l3.next = l1;l1 = l1.next;}else{l3.next = l2;l2 = l2.next;}l3 = l3.next;}if(l1 != null){l3.next = l1;}if(l2 != null){l3.next = l2;}return flag.next;
}

}