Lintcode: Hash Function && Summary: Modular Multiplication, Addition, Power && Summary: 长整形long

In data structure Hash, hash function is used to convert a string(or any other type) into an integer smaller than hash size and bigger or equal to zero. The objective of designing a hash function is to "hash" the key as unreasonable as possible. A good hash function can avoid collision as less as possible. A widely used hash function algorithm is using a magic number 33, consider any string as a 33 based big integer like follow:

hashcode("abcd") = (ascii(a) * 333 + ascii(b) * 332 + ascii(c) *33 + ascii(d)) % HASH_SIZE 

                              = (97* 33^3 + 98 * 33^2 + 99 * 33 +100) % HASH_SIZE

                              = 3595978 % HASH_SIZE

here HASH_SIZE is the capacity of the hash table (you can assume a hash table is like an array with index 0 ~ HASH_SIZE-1).

Given a string as a key and the size of hash table, return the hash value of this key.f



Example
For key="abcd" and size=100, return 78

Clarification
For this problem, you are not necessary to design your own hash algorithm or consider any collision issue, you just need to implement the algorithm as described.

 1 class Solution {
 2     /**
 3      * @param key: A String you should hash
 4      * @param HASH_SIZE: An integer
 5      * @return an integer
 6      */
 7     public int hashCode(char[] key,int HASH_SIZE) {
 8         if (key.length == 0) return 0;
 9         int result = 0;
10         int base = 1;
11         for (int i=key.length-1; i>=0; i--) {
12             int num = (int)(key[i] - ‘\0‘);
13             result += modMultiply(num, base, HASH_SIZE);
14             result = result % HASH_SIZE;
15             base = modMultiply(base, 33, HASH_SIZE);
16         }
17         return result % HASH_SIZE;
18     }
19     
20     public int modMultiply(int a, int b, int c) {
21         long temp = (long)a * b;
22         return (int)(temp % c);
23     }
24 };

1 public int modMultiply(int a, int b, int c) {
2          long temp = a * b;
3          return (int)(temp % c);
4      }
5 };

 1     public int hashCode(char[] key,int HASH_SIZE) {
 2         int result = 0;
 3         for (int i = 0; i < key.length; i++) {
 4             result = helper(result, 33, HASH_SIZE);
 5             result += key[i];
 6             result %= HASH_SIZE;
 7         }
 8         return result;
 9     }
10 
11     int helper(int num, int base, int mod) {
12         int result = 0;
13         int temp = num - mod;
14         for (int i = 0; i < base; i++) {
15             if (result + temp > 0) {
16                 result += temp;
17             } else {
18                 result += num;
19             }
20         }
21         return result;
22     }