哈希表定义及实现

哈希表也叫散列表，是快速执行查找，删除，插入的技术，不支持元素排序信息。原理是通过把关键码值映射到表中一个位置来访问记录，以加快查找的速度。
关键码值到存储位置的映射被称为哈希函数也叫散列函数，当不同关键 key 被映射到同一 value 时，称为冲突。
解决冲突主要有：
分离链接法：对 Hash 表中每个 Hash 值建立一个冲突表，即将冲突的几个记录以表的形式存储在其中。
开发定址法：根据冲突算法找到空的单元为止（线性探测法，平方探测法 …）。

编程实例

算法实现

采用平方探测法的实现放在 git@code.aliyun.com:c-program/projecteuler.git 上 projecteulerccommonstruct 目录下 elr_hash_int.h elr_hash_int.c 两个单元内。

解决问题

https://www.hackerrank.com/ch…
要求用一个 map 结构存放名字 name 和电话 phone。

定义结构

#define MaxStr 20

typedef unsigned long int Index;
typedef Index Position;
typedef char *KeyType;
typedef long int *ValueType;

struct HashTbl;
typedef struct HashTbl *HashTable;

enum KindOfEntry {Legitimate, Empty, Deleted};

struct HashEntry {
  KeyType Key;
  ValueType Value;
  enum KindOfEntry Info;
};

typedef struct HashEntry Cell;

struct HashTbl {
  long int TableSize;
  Cell *TheCells;
};

初始化及释放

HashTable InitializeTable(long int TableSize) {
  HashTable H;
  long int i;

  H = malloc(sizeof(struct HashTbl));
  if (H == NULL) {printf("Out of space!\n");
  }

  H->TableSize = TableSize;

  H->TheCells = malloc(sizeof(Cell) * H->TableSize);
  if (H->TheCells == NULL) {printf("Out of space!\n");
  }

  for (i = 0; i < H->TableSize; i++) {H->TheCells[i].Info = Empty;
    H->TheCells[i].Key = NULL;
    H->TheCells[i].Value = 0;
  }

  return H;
}

void DestroyTable(HashTable H) {
  int i;

  if (H == NULL) {return;}

  for (i = 0; i < H->TableSize; i++) {H->TheCells[i].Info = Empty;
    if (H->TheCells[i].Key != NULL) {free(H->TheCells[i].Key);
    }
  }

  free(H);
}

哈希及冲突插入操作

Position Hash(KeyType Key, long int TableSize) {
  long int keyValue = 0;
  register unsigned char *p;

  for (p = (unsigned char *)Key; *p; p++) {keyValue = 31 * keyValue + *p;}

  return keyValue % TableSize;
}

int KeyEqual(KeyType a, KeyType b) {if ((a == NULL) || (b == NULL)) {return 1;}

  return strcmp(a, b) == 0 ? 0 : 1;
}

int Find(KeyType Key, HashTable H, Position *Pos) {
  Position CurrentPos;
  long int CollisionNum;
  int rst = 0;

  CollisionNum = 0;
  CurrentPos = Hash(Key, H->TableSize);
  while (1) {if (H->TheCells[CurrentPos].Info == Empty)
      break;
    if (KeyEqual(H->TheCells[CurrentPos].Key, Key) == 0) {
      rst = 1;
      break;
    }
    CurrentPos += (++CollisionNum << 1) - 1;
    if (CurrentPos >= H->TableSize) {CurrentPos -= H->TableSize;}
  }

  *Pos = CurrentPos;
  return rst;
}

void Insert(KeyType Key, ValueType Value, HashTable H) {
  Position Pos;
  int exist;

  exist = Find(Key, H, &Pos);

  H->TheCells[Pos].Info = Legitimate;
  if (!exist) {H->TheCells[Pos].Key = malloc(sizeof(char) * MaxStr);
  }
  strcpy(H->TheCells[Pos].Key, Key);
  H->TheCells[Pos].Value = Value;
}

调用解决问题

#include <math.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main() {
  long int n = 0;
  scanf("%ld", &n);

  HashTable H = InitializeTable(n * 2);

  for (int i = 0; i < n; i++) {char name[20];
    long int phone;
    scanf("%s %ld", name, &phone);
    Insert(name, phone, H);
    }

    for (int i = 0; i < n; i++) {char name[20];
      Position Pos;
      int exist;
      scanf("%s", name);
      // while (() != '\n');
      exist = Find(name, H, &Pos);
      if (exist == 1) {ValueType Value = H->TheCells[Pos].Value;
          printf("%s=%ld\n", name, Value);
      } else {printf("Not found\n");
      }
      if (getchar() == EOF)
        break;
    }

    DestroyTable(H);
    
    return 0;
}