LintCode: Hash Function

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
        int len = key.length();
        int magic = 1;
        long sum = (int)key[0];
        for (int i = 1; i < len; i++) {
            sum = (sum*33)%HASH_SIZE + (int)key[i];
        }
        return (int)sum%HASH_SIZE;
    }
};