1.反转链表
public ListNode ReverseList (ListNode head) {
if (head==null ){
return null ;
}
ListNode newHead = new ListNode(-1);
ListNode cur = head;
while (cur!=null ){
ListNode next = cur.next;
cur.next = newHead.next;
newHead.next = cur;
cur = next;
}
return newHead.next;
}
public ListNode ReverseList (ListNode head) {
if(head==null||head.next==null){
return head;
}
ListNode newHead = ReverseList(head.next);
head.next.next = head;
head.next = null;
return newHead;
}
指定区间内反转链表
public ListNode reverse(ListNode head,ListNode end){
if(head==null||head.next ==null)
return head;
ListNode newHead = new ListNode(-1);
ListNode cur = head;
while(cur!=end){
ListNode next = cur.next;
cur.next = newHead.next;
newHead.next = cur;
cur = next;
}
return newHead.next;
}
public ListNode reverseBetween (ListNode head, int m, int n) {
ListNode newHead = new ListNode(-1);
newHead.next = head;
ListNode prev = newHead;
ListNode cur = head;
for(int i = 0;i <m-1;i++){
prev = prev.next;
cur = cur.next;
}
ListNode next = head;
ListNode tmp = cur;
for(int i = 0;i<n;i++){
next = next.next;
}
ListNode ret = reverse(cur,next);
prev.next = ret;
tmp.next = next;
return newHead.next;
}
- 合并两个排序的链表(2种方法)
public ListNode Merge (ListNode pHead1, ListNode pHead2) {
ListNode newHead = new ListNode(-1);
ListNode tail = newHead;
while(pHead1!=null&&pHead2!=null){
if(pHead1.val<pHead2.val){
tail.next = pHead1;
pHead1=pHead1.next;
tail=tail.next;
}else{
tail.next = pHead2;
pHead2=pHead2.next;
tail=tail.next;
}
}
while(pHead1!=null){
tail.next=pHead1;
pHead1=pHead1.next;
tail=tail.next;
}
while(pHead2!=null){
tail.next=pHead2;
pHead2=pHead2.next;
tail=tail.next;
}
return newHead.next;
}
public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
if(list1==null)
return list2;
if(list2==null)
return list1;
if(list1.val<list2.val){
list1.next = mergeTwoLists(list1.next,list2);
return list1;
}else{
list2.next = mergeTwoLists(list1,list2.next);
return list2;
}
}
4.合并k个已排序的链表(2种方法)
public ListNode mergeKLists (ArrayList<ListNode> lists){
return merge(lists,0,lists.size()-1);
}
public ListNode merge(ArrayList<ListNode> lists,int left, int right){
if(left>right)
return null;
if(left==right)
return lists.get(left);
int mid = (left+right)/2;
ListNode l1 = merge(lists,left,mid);
ListNode l2 = merge(lists,mid+1,right);
return Merge(l1,l2);
}
public ListNode mergeKLists2 (ArrayList<ListNode> lists){
PriorityQueue<ListNode> heap = new PriorityQueue<>((v1,v2)->v1.val-v2.val);
for(ListNode l:lists){
if(l!=null)
heap.offer(l);
}
ListNode newHead = new ListNode(-1);
ListNode cur = newHead;
while(!heap.isEmpty()){
ListNode node = heap.poll();
cur.next = node;
cur = cur.next;
if(node.next!=null)
heap.offer(node.next);
}
return newHead.next;
}
//=======================================================
public ListNode Merge (ListNode pHead1, ListNode pHead2) {
ListNode newHead = new ListNode(-1);
ListNode tail = newHead;
while(pHead1!=null&&pHead2!=null){
if(pHead1.val<pHead2.val){
tail.next = pHead1;
pHead1=pHead1.next;
tail=tail.next;
}else{
tail.next = pHead2;
pHead2=pHead2.next;
tail=tail.next;
}
}
while(pHead1!=null){
tail.next=pHead1;
pHead1=pHead1.next;
tail=tail.next;
}
while(pHead2!=null){
tail.next=pHead2;
pHead2=pHead2.next;
tail=tail.next;
}
return newHead.next;
}
5.判断是否有环
public boolean hasCycle(ListNode head) {
if(head==null){
return false;
}
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;
}