9.1---上楼梯（CC150）

    
    public static int countWays(int n){
        if(n == 1) return 1;
        if(n == 2) return 2;
        if(n == 3) return 4;
        long[] dp = new long[n];
        dp[0] = 1;
        dp[1] = 2;
        dp[2] = 4;
        for(int i = 3; i < n; i++){
            dp[i] = (dp[i-1] + dp[i-2] + dp[i-3])%1000000007;
        }
        return (int) (dp[n - 1] % 1000000007);
    }