hash
hash
Goal: For faster search—O(1), exchange space for time, and calculate the spatial position faster
definition
A collection of "key-value pairs"
two key questions
-
Hash Function What is a good hash function
-
how to resolve conflict
conflict resolution
-
open addressing
-
split chain
-
other (public overflow)
open addressing
h(x) = x mod 11
x={12,2,17,28,6,23} h(x) = {1,2,6,6,1}
In this way, there will be conflicts. When there is a conflict, the data can be put into the subsequent coordinate points for linear detection and square detection. . .
split chain
h(x) = x mod 11
x={12,2,17,28,6,23} h(x) = {1,2,6,6,1}
It is to make a linked list node at this node. Insertion and deletion of singly linked list
C implementation
// 开放定址法，删除时时不能真正的删，只能逻辑的删除，让其标记
#include <stdio.h>
#include <stdio.h>
// 标记的状态
enum GridStatus{
Active,Removed,Empty};
typedef enum GridStatus Status;
struct HashTable{
int *key; // key
Status *status; // 标记的状体
int size; // 表里的大小
int remains; // 表中剩余的大小
};
typedef struct HashTable *HT;
HT initHashTable(int size){
HT h;
h= (HT) malloc(sizeof(struct HashTable));
if(!h) return NULL;
h->size = h->remains =size;
h->key = (int *)malloc(sizeof(Status)* size);
if(!h->status){
free(h->key);free(h); return NUll;}
for(int i=0;i<size;i++){
h->status[i]=Empty;
}
return h;
}
// hash函数
int hash(int x,int p){
return x%p;
}
int isFull(const HT h){
return h->remains == 0;
}
// 插入
int insertX(int x, HT h){
if(isFull(h)) return 0;
int pos=hash(x,h->size);
while(h->status[pos] == Active){
pos = (pos+1) % h -> size;
}
h->key[pos] = x;
h->status[pos] =Active;
h->remains--;
return 1;
}
// 查找
int findX(int x,HT h){
int pos,index;
index=pos= hash(x,h->size);
while(h-status[pos]!=Empty){
if(h->key[pos] == x && h-status[pos] == Active) return pos;
pos= (pos+1)% h->size; // 循环一圈
if(pos == index) break;
return -1;
}
}
//remove
int removeX(int x, HT h){
int pos = findX(x,h);
if(pos==-1) return 0;
// 只做标记，不修改其中的值，这是绝大部分就是这样的。
h->status[pos] = Empty;
}
void printHT(const HT h){
for(int i=0;i<h-size;i++){
printf("%4d",i){
}
putchar('\n');
for(int i=0;i<h-size;i++){
if(h->status[i] == Active) printf("%4d",h->key[i]);
else if(h->status[i]==Removed) printf(" X");
else printf(" -");
}
putchar('\n');
}
int main(){
HT h=initHashTable(11);
insertX(5,h);
return 0;
}
C++
#include <iostream>
#include <map>
#include <set>
#include <unordered_map>
#include <unordered_set>
using namespace std;
int main() {
unordered_set<int> s;
unordered_map<int, int> m;
m[101] =33;
m[-10] = 2233;
m.inser(pair<int,int>(22,33));
return 0;
}
Java
Map-->HashMap; 键值对映射
Set--->HashSet;
The hashmap in JAVA is an array + linked list before jdk1.8, and an array + linked list + red-black tree is used after jdk1.8. When the number of elements is 8, the linked list is converted into a red-black tree, so When the number of hashmap elements is around 8, performance will be consumed. The number of initial value elements of hashMap is 16, the self-increment factor of hashMap is 0.75, and when the number of elements is 32, the initial capacity is 32/0.75=.
Python
d = dict() # 字典
d[100] =50