DP问题：f(n) = 2f(n-1)

public class Solution {
    public int jumpFloorII(int target) {
        int result = 0;
        int[] dp = new int[2];
        dp[0] = 1;
        if (1 == target) {
            return 1;
        }
        for(int i = 1; i < target; i++) {
            result = 2 * dp[0];
            dp[0] = result;
        }
        return result;
    }
}