2024年7月6日
兩兩交換連結串列元素
繼續用虛擬頭節點，多建立幾個記錄指標。
題24. 兩兩交換連結串列中的節點
/**
* 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) {
if(head==null){
return null;
}
if(head.next==null){
return head;
}
ListNode nextSpan = head.next.next;
ListNode vir = new ListNode();
vir.next = head;
ListNode before = vir;
while(true){
ListNode cur1 = before.next;
ListNode cur2 = cur1.next;
before.next = cur2;
cur2.next = cur1;
cur1.next = nextSpan;
before = cur1;
cur1 = before.next;
if(cur1==null||cur1.next==null){
break;
}
cur2 = cur1.next;
nextSpan = cur2.next;
}
return vir.next;
}
}
題19. 刪除連結串列的倒數第n個節點
雙指標法
第二個指標先移動n+1個，當到末尾時，第一個指標的下一個就刪除。
/**
* 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 vir = new ListNode();
vir.next = head;
ListNode fast = vir;
ListNode slow = vir;
for(int i=0;i<=n;i++){
fast = fast.next;
}
while(fast!=null){
slow = slow.next;
fast = fast.next;
}
slow.next = slow.next.next;
return vir.next;
}
}
面試題 02.07. 連結串列相交
求出長度差值，然後長的切掉多出來的，然後依次比較節點是否相同。
/**
* Definition for singly-linked list.
* public class ListNode {
* int val;
* ListNode next;
* ListNode(int x) {
* val = x;
* next = null;
* }
* }
*/
public class Solution {
public ListNode getIntersectionNode(ListNode headA, ListNode headB) {
if(headA==null||headB==null){
return null;
}
ListNode p1 = headA,p2=headB;
int a=1,b=1;
while(p1.next!=null){
p1 = p1.next;
a+=1;
}
while(p2.next!=null){
p2=p2.next;
b+=1;
}
p1 = headA;
p2 = headB;
int cha = Math.abs(a-b);
if(a>b){
for(int i=0;i<cha;i++){
headA = headA.next;
}
}else{
for(int i=0;i<cha;i++){
headB = headB.next;
}
}
p1 = headA;
p2 = headB;
while(p1!=null || p2!=null){
if(p1==p2){
return p1;
}else{
p1 = p1.next;
p2 = p2.next;
}
}
return null;
}
}
環形連結串列
不需要虛擬頭節點，快慢指標，相交之後，從相交點和head到入環位置的距離相同。
/**
* Definition for singly-linked list.
* class ListNode {
* int val;
* ListNode next;
* ListNode(int x) {
* val = x;
* next = null;
* }
* }
*/
public class Solution {
public ListNode detectCycle(ListNode head) {
if(head==null){
return null;
}
ListNode fast=head,slow=head;
do{
fast=fast.next;
if(fast==null){
return null;
}
fast = fast.next;
slow = slow.next;
if(fast==null){
return null;
}
}while(fast!=slow);
fast = head;
while(fast!=slow){
fast = fast.next;
slow = slow.next;
}
return fast;
}
}