05 August 2008
单链表
时间复杂度： addAtHead： O(1) addAtInder，get，deleteAtIndex: O(k)，其中 k 指的是元素的索引。 addAtTail：O(N)，其中 N 指的是链表的元素个数。
空间复杂度：所有的操作都是 O(1)O(1)。
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public class ListNode {
int val;
ListNode next;
ListNode(int x) { val = x; }
}
class MyLinkedList {
int size;
ListNode head; // sentinel node as pseudo-head
public MyLinkedList() {
size = 0;
head = new ListNode(0);
}
/** Get the value of the index-th node in the linked list. If the index is invalid, return -1. */
public int get(int index) {
// if index is invalid
if (index < 0 || index >= size) return -1;
ListNode curr = head;
// index steps needed
// to move from sentinel node to wanted index
for(int i = 0; i < index + 1; ++i) curr = curr.next;
return curr.val;
}
/** Add a node of value val before the first element of the linked list. After the insertion, the new node will be the first node of the linked list. */
public void addAtHead(int val) {
addAtIndex(0, val);
}
/** Append a node of value val to the last element of the linked list. */
public void addAtTail(int val) {
addAtIndex(size, val);
}
/** Add a node of value val before the index-th node in the linked list. If index equals to the length of linked list, the node will be appended to the end of linked list. If index is greater than the length, the node will not be inserted. */
public void addAtIndex(int index, int val) {
// If index is greater than the length,
// the node will not be inserted.
if (index > size) return;
// [so weird] If index is negative,
// the node will be inserted at the head of the list.
if (index < 0) index = 0;
++size;
// find predecessor of the node to be added
ListNode pred = head;
for(int i = 0; i < index; ++i) pred = pred.next;
// node to be added
ListNode toAdd = new ListNode(val);
// insertion itself
toAdd.next = pred.next;
pred.next = toAdd;
}
/** Delete the index-th node in the linked list, if the index is valid. */
public void deleteAtIndex(int index) {
// if the index is invalid, do nothing
if (index < 0 || index >= size) return;
size--;
// find predecessor of the node to be deleted
ListNode pred = head;
for(int i = 0; i < index; ++i) pred = pred.next;
// delete pred.next
pred.next = pred.next.next;
}
}
双链表
时间复杂度： addAtHead，addAtTail：O(1) get，addAtIndex，delete：O(min(k,N−k))，其中 k 指的是元素的索引。
空间复杂度：所有的操作都是 O(1)。
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public class ListNode {
int val;
ListNode next;
ListNode prev;
ListNode(int x) { val = x; }
}
class MyLinkedList {
int size;
// sentinel nodes as pseudo-head and pseudo-tail
ListNode head, tail;
public MyLinkedList() {
size = 0;
head = new ListNode(0);
tail = new ListNode(0);
head.next = tail;
tail.prev = head;
}
/** Get the value of the index-th node in the linked list. If the index is invalid, return -1. */
public int get(int index) {
// if index is invalid
if (index < 0 || index >= size) return -1;
// choose the fastest way: to move from the head
// or to move from the tail
ListNode curr = head;
if (index + 1 < size - index)
for(int i = 0; i < index + 1; ++i) curr = curr.next;
else {
curr = tail;
for(int i = 0; i < size - index; ++i) curr = curr.prev;
}
return curr.val;
}
/** Add a node of value val before the first element of the linked list. After the insertion, the new node will be the first node of the linked list. */
public void addAtHead(int val) {
ListNode pred = head, succ = head.next;
++size;
ListNode toAdd = new ListNode(val);
toAdd.prev = pred;
toAdd.next = succ;
pred.next = toAdd;
succ.prev = toAdd;
}
/** Append a node of value val to the last element of the linked list. */
public void addAtTail(int val) {
ListNode succ = tail, pred = tail.prev;
++size;
ListNode toAdd = new ListNode(val);
toAdd.prev = pred;
toAdd.next = succ;
pred.next = toAdd;
succ.prev = toAdd;
}
/** Add a node of value val before the index-th node in the linked list. If index equals to the length of linked list, the node will be appended to the end of linked list. If index is greater than the length, the node will not be inserted. */
public void addAtIndex(int index, int val) {
// If index is greater than the length,
// the node will not be inserted.
if (index > size) return;
// [so weird] If index is negative,
// the node will be inserted at the head of the list.
if (index < 0) index = 0;
// find predecessor and successor of the node to be added
ListNode pred, succ;
if (index < size - index) {
pred = head;
for(int i = 0; i < index; ++i) pred = pred.next;
succ = pred.next;
}
else {
succ = tail;
for (int i = 0; i < size - index; ++i) succ = succ.prev;
pred = succ.prev;
}
// insertion itself
++size;
ListNode toAdd = new ListNode(val);
toAdd.prev = pred;
toAdd.next = succ;
pred.next = toAdd;
succ.prev = toAdd;
}
/** Delete the index-th node in the linked list, if the index is valid. */
public void deleteAtIndex(int index) {
// if the index is invalid, do nothing
if (index < 0 || index >= size) return;
// find predecessor and successor of the node to be deleted
ListNode pred, succ;
if (index < size - index) {
pred = head;
for(int i = 0; i < index; ++i) pred = pred.next;
succ = pred.next.next;
}
else {
succ = tail;
for (int i = 0; i < size - index - 1; ++i) succ = succ.prev;
pred = succ.prev.prev;
}
// delete pred.next
--size;
pred.next = succ;
succ.prev = pred;
}
}