leetcode1987 不同的好子序列数目

思路：

动态规划，参考了https://leetcode-cn.com/problems/number-of-unique-good-subsequences/solution/bu-tong-de-hao-zi-xu-lie-shu-mu-by-leetc-ej2n/。

实现：

class Solution
{
public:
    int numberOfUniqueGoodSubsequences(string binary)
    {
        int n = binary.length(), mod = 1e9 + 7;
        int res = 0, a = 0, b = 0;
        for (int i = 1; i <= n; i++)
        {
            int new_a = 0, new_b = 0;
            if (binary[i - 1] == '1')
            {
                new_a = a;
                new_b = (a + b + 1) % mod;
            }
            else 
            {
                if (res == 0) res++;
                new_a = (a + b) % mod;
                new_b = b;
            }
            a = new_a; b = new_b;
        }
        res = (res + a + b) % mod;
        return res;
    }
};