LintCode_128 Hash Function

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* 333 + 98 * 332 + 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

要注意的问题是计算过程中可能数值过大超出int的范围， 所以要用long来实现，被操作数一定要用long来实现，否则两个int类型进行计算，可能先溢出了，

再赋值给long，结果还是溢出的结果，long并没有起作用。

class Solution {public:    /**     * @param key: A String you should hash     * @param HASH_SIZE: An integer     * @return an integer     */    int hashCode(string key,int HASH_SIZE) {        // write your code here        if (key.size() == 0)              return 0;        int len = (int)key.size();        long factor = 1;        long res = 0;        for (int i = len - 1 ; i >= 0; i--) {            if (i == len - 1) {                res+=key[i] % HASH_SIZE;                continue;            }            factor = (factor * 33) % HASH_SIZE;            res+=(key[i] * factor) % HASH_SIZE;            //cout<<key[i]<<" "<<(key[i] * factor) % HASH_SIZE<<endl;            //cout<<res<<endl;        }        return res % HASH_SIZE;    }};