认识哈希函数和哈希表的实现
MD5 表达16^16范围的值 SHal 表达16^32范围的值
输入相同，即输出相同，不随机
不同的输出，输出相同
均匀性，离散性
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map.Entry;
public class HashMap {
public static void main(String[] args) {
HashMap<String, String> map = new HashMap<>();
map.put("zuo", "31");
System.out.println(map.containsKey("zuo"));
System.out.println(map.containsKey("chengyun"));
System.out.println(map.get("zuo"));
System.out.println(map.get("chengyun"));
System.out.println(map.isEmpty());
System.out.println(map.size());
System.out.println(map.remove("zuo"));
System.out.println(map.containsKey("zuo"));
System.out.println(map.get("zuo"));
System.out.println(map.isEmpty());
System.out.println(map.size());
map.put("zuo", "31");
System.out.println(map.get("zuo"));
map.put("zuo", "32");
System.out.println(map.get("zuo"));
map.put("zuo", "31");
map.put("cheng", "32");
map.put("yun", "33");
for (String key : map.keySet()) {
System.out.println(key);
}
for (String values : map.values()) {
System.out.println(values);
}
map.clear();
map.put("A", "1");
map.put("B", "2");
map.put("C", "3");
map.put("D", "1");
map.put("E", "2");
map.put("F", "3");
map.put("G", "1");
map.put("H", "2");
map.put("I", "3");
for (Entry<String, String> entry : map.entrySet()) {
String key = entry.getKey();
String value = entry.getValue();
System.out.println(key + "," + value);
}
// you can not remove item in map when you use the iterator of map
// for(Entry<String,String> entry : map.entrySet()){
// if(!entry.getValue().equals("1")){
// map.remove(entry.getKey());
// }
// }
// if you want to remove items, collect them first, then remove them by
// this way.
List<String> removeKeys = new ArrayList<String>();
for (Entry<String, String> entry : map.entrySet()) {
if (!entry.getValue().equals("1")) {
removeKeys.add(entry.getKey());
}
}
for (String removeKey : removeKeys) {
map.remove(removeKey);
}
for (Entry<String, String> entry : map.entrySet()) {
String key = entry.getKey();
String value = entry.getValue();
System.out.println(key + "," + value);
}
}
}
设计RandomPool结构
设计一种结构，在该结构中有如下三个功能：
insert (key):将某个key加入到该结构，做到不重复加入 delete (key):将原本在结构中的某个key移除 getRandom():等概率随机返回结构中的任何一个key。
【要求】
Insert、 delete和getRandom方法的时间复杂度都是0 (1)
import java.util.HashMap;
public class RandomPool {
public static class Pool<K> {
private HashMap<K, Integer> keyIndexMap;
private HashMap<Integer, K> indexKeyMap;
private int size;
public Pool() {
this.keyIndexMap = new HashMap<K, Integer>();
this.indexKeyMap = new HashMap<Integer, K>();
this.size = 0;
}
public void insert(K key) {
if (!this.keyIndexMap.containsKey(key)) {
this.keyIndexMap.put(key, this.size);
this.indexKeyMap.put(this.size++, key);
}
}
public void delete(K key) {
if (this.keyIndexMap.containsKey(key)) {
int deleteIndex = this.keyIndexMap.get(key);
int lastIndex = --this.size;
K lastKey = this.indexKeyMap.get(lastIndex);
this.keyIndexMap.put(lastKey, deleteIndex);
this.indexKeyMap.put(deleteIndex, lastKey);
this.keyIndexMap.remove(key);
this.indexKeyMap.remove(lastIndex);
}
}
public K getRandom() {
if (this.size == 0) {
return null;
}
int randomIndex = (int) (Math.random() * this.size); // 0 ~ size -1
return this.indexKeyMap.get(randomIndex);
}
}
public static void main(String[] args) {
Pool<String> pool = new Pool<String>();
pool.insert("zuo");
pool.insert("cheng");
pool.insert("yun");
System.out.println(pool.getRandom());
System.out.println(pool.getRandom());
System.out.println(pool.getRandom());
System.out.println(pool.getRandom());
System.out.println(pool.getRandom());
System.out.println(pool.getRandom());
}
}
详解布隆过滤器
n=样本量 p=失误率
m = -n * ln p / (ln 2)^2 空间大小
k = ln 2 * m/n 约等于 0.7 * m/n
p真 = (1 - e ^ (-n * k真 / m真))^k真
位图
public class BitMap {
public static void main(String[] args) {
int a = 0;
// a 32 bit
int[] arr = new int[10]; // 32bit * 10 -> 320 bits
// arr[0] int 0 ~ 31
// arr[1] int 32 ~ 63
// arr[2] int 64 ~ 95
int i = 178; // 想取得178个bit的状态
int numIndex = i / 32;
int bitIndex = i % 32;
// 拿到178位的状态
int s = ( (arr[numIndex] >> (bitIndex)) & 1);
// 请把178位的状态改成1
arr[numIndex] = arr[numIndex] | (1 << (bitIndex));
i = 178; // 请把178位的状态改成0
arr[numIndex] = arr[numIndex] & (~ (1 << bitIndex) );
i = 178; // 请把178位的状态拿出来
// bit 0 1
int bit = (arr[i / 32] >> (i % 32)) & 1;
}
}
详解一致性哈希原理
https://www.jianshu.com/p/e968c081f563
岛问题
一个矩阵中只有。和1两种值，每个位置都可以和自己的上、下、左、右 四个位置相连，如 果有一片1连在一起，这个部分叫做一个岛，求一个矩阵中有多少个岛？
【举例】
001010
111010 100100 000000
这个矩阵中有三个岛 【进阶】如何设计一个并行算法解决这个问题
public class Code03_Islands {
public static int countIslands(int[][] m) {
if (m == null || m[0] == null) {
return 0;
}
int N = m.length;
int M = m[0].length;
int res = 0;
for (int i = 0; i < N; i++) {
for (int j = 0; j < M; j++) {
if (m[i][j] == 1) {
res++;
infect(m, i, j, N, M);
}
}
}
return res;
}
public static void infect(int[][] m, int i, int j, int N, int M) {
if (i < 0 || i >= N || j < 0 || j >= M || m[i][j] != 1) {
return;
}
// i，j没越界，并且当前位置值是1
m[i][j] = 2;
infect(m, i + 1, j, N, M);
infect(m, i - 1, j, N, M);
infect(m, i, j + 1, N, M);
infect(m, i, j - 1, N, M);
}
public static void main(String[] args) {
int[][] m1 = { { 0, 0, 0, 0, 0, 0, 0, 0, 0 },
{ 0, 1, 1, 1, 0, 1, 1, 1, 0 },
{ 0, 1, 1, 1, 0, 0, 0, 1, 0 },
{ 0, 1, 1, 0, 0, 0, 0, 0, 0 },
{ 0, 0, 0, 0, 0, 1, 1, 0, 0 },
{ 0, 0, 0, 0, 1, 1, 1, 0, 0 },
{ 0, 0, 0, 0, 0, 0, 0, 0, 0 }, };
System.out.println(countIslands(m1));
int[][] m2 = { { 0, 0, 0, 0, 0, 0, 0, 0, 0 },
{ 0, 1, 1, 1, 1, 1, 1, 1, 0 },
{ 0, 1, 1, 1, 0, 0, 0, 1, 0 },
{ 0, 1, 1, 0, 0, 0, 1, 1, 0 },
{ 0, 0, 0, 0, 0, 1, 1, 0, 0 },
{ 0, 0, 0, 0, 1, 1, 1, 0, 0 },
{ 0, 0, 0, 0, 0, 0, 0, 0, 0 }, };
System.out.println(countIslands(m2));
}
}
并查集结构的详解和实现
import java.util.HashMap;
import java.util.List;
import java.util.Stack;
public class Code04_UnionFind {
// 样本进来会包一层，叫做元素
public static class Element<V> {
public V value;
public Element(V value) {
this.value = value;
}
}
public static class UnionFindSet<V> {
public HashMap<V, Element<V>> elementMap;
// key 某个元素 value 该元素的父
public HashMap<Element<V>, Element<V>> fatherMap;
// key 某个集合的代表元素 value 该集合的大小
public HashMap<Element<V>, Integer> sizeMap;
public UnionFindSet(List<V> list) {
elementMap = new HashMap<>();
fatherMap = new HashMap<>();
sizeMap = new HashMap<>();
for (V value : list) {
Element<V> element = new Element<V>(value);
elementMap.put(value, element);
fatherMap.put(element, element);
sizeMap.put(element, 1);
}
}
// 给定一个ele，往上一直找，把代表元素返回
private Element<V> findHead(Element<V> element) {
Stack<Element<V>> path = new Stack<>();
while (element != fatherMap.get(element)) {
path.push(element);
element = fatherMap.get(element);
}
while (!path.isEmpty()) {
fatherMap.put(path.pop(), element);
}
return element;
}
public boolean isSameSet(V a, V b) {
if (elementMap.containsKey(a) && elementMap.containsKey(b)) {
return findHead(elementMap.get(a)) == findHead(elementMap.get(b));
}
return false;
}
public void union(V a, V b) {
if (elementMap.containsKey(a) && elementMap.containsKey(b)) {
Element<V> aF = findHead(elementMap.get(a));
Element<V> bF = findHead(elementMap.get(b));
if (aF != bF) {
Element<V> big = sizeMap.get(aF) >= sizeMap.get(bF) ? aF : bF;
Element<V> small = big == aF ? bF : aF;
fatherMap.put(small, big);
sizeMap.put(big, sizeMap.get(aF) + sizeMap.get(bF));
sizeMap.remove(small);
}
}
}
}
}