LeetCode 372. Super Pow

原题网址：https://leetcode.com/problems/super-pow/

Your task is to calculate a^b mod 1337 where a is a positive integer and b is an extremely large positive integer given in the form of an array.

Example1:

a = 2
b = [3]

Result: 8

Example2:

a = 2
b = [1,0]

Result: 1024

方法：分析模的特点，若a % 1337 = n * 1337 + m，b % 1337 = p * 1337 + q，则a*b % 1337 = m * q % 1337

public class Solution {
    private int[] mod;
    private int mod(int a, int p) {
        if (mod[p] > 0) return mod[p];
        if (p == 0) {
            mod[p] = a % 1337;
            return mod[p];
        }
        int m = 1;
        for(int i = 0; i < 10; i++) {
            m *= mod(a, p - 1);
            m %= 1337;
        }
        mod[p] = m;
        return mod[p];
    }
    public int superPow(int a, int[] b) {
        if (a % 191 == 0 && a % 7 == 0) return 0;
        mod = new int[b.length];
        int m = 1;
        for(int i = 0; i < b.length; i++) {
            for(int j = 1; j <= b[i]; j++) {
                m *= mod(a, b.length - 1 - i);
                m %= 1337;
            }
        }
        return m;
    }
}