变态跳台阶

f(n-1) = f(n-2) + f(n-3) + ... + f(0)

f(n) = f(n-1) + f(n-2) + ... + f(0)

f(n) - f(n-1) = f(n-1)

f(n) = 2*f(n-1)

public int JumpFloorII(int target) {
    return (int) Math.pow(2, target - 1);
}

public int jumpFloorII(int target) {
    int[] dp = new int[target];
    Arrays.fill(dp, 1);
    for (int i = 1; i < target; i++)
        for (int j = 0; j < i; j++)
            dp[i] += dp[j];
    return dp[target - 1];
}