奇妙序列

/* 
 * 题目思路：
 * 与Java代码相同，只是这里使用Java Stream处理append操作。 
 * 我们还是延迟更新策略，在getIndex时计算实际值。
 */
import java.util.ArrayList;
import java.util.stream.IntStream;

public class Fancy {
    private static final int MOD = 1_000_000_007;
    private ArrayList<Integer> sequence;
    private int add;
    private int mul;

    public Fancy() {
        sequence = new ArrayList<>();
        add = 0;
        mul = 1;
    }

    public void append(int val) {
        val = (int)((val - add + MOD) % MOD);
        val = (int)((val * modInverse(mul, MOD)) % MOD);
        sequence.add(val);
    }

    public void addAll(int inc) {
        add = (int)((add + inc) % MOD);
    }

    public void multAll(int m) {
        mul = (int)((mul * m) % MOD);
        add = (int)((add * m) % MOD);
    }

    public int getIndex(int idx) {
        if (idx >= sequence.size()) {
            return -1;
        }
        return (int)(((long)sequence.get(idx) * mul + add) % MOD);
    }

    private int modInverse(int a, int m) {
        return power(a, m - 2, m);
    }

    private int power(int x, int y, int p) {
        int res = 1;
        x = x % p;
        while (y > 0) {
            if ((y & 1) == 1)
                res = (res * x) % p;
            y = y >> 1;
            x = (x * x) % p;
        }
        return res;
    }
}