1955

class Solution {
public:
    int countSpecialSubsequences(vector<int>& nums) {
        long long int dp[3];
        memset(dp, 0, sizeof dp);
        long long mod = int(1e9)+7;
        for(int i=0; i<nums.size(); i++){
            int a = nums[i];
            if(a == 0)
                dp[a] = (2LL*dp[a]+1)%mod;
            else
                dp[a] = (2LL*dp[a]+dp[a-1])%mod;
        }
        return dp[2];
    }
};