剑指offer - 变态跳台阶

public class Solution {
    public int JumpFloorII(int target) {
        if (target <= 0) {
            return 0;
        } else if (target == 1) {
            return 1;
        } else {
            return 2 * JumpFloorII(target - 1);
        }
    }
}

public class Solution {
    public int JumpFloorII(int target) {
        if (target <= 0) {
            return 0;
        }
        if (target == 1) {
            return 1;
        }
        int x = 1;
        int y = 2;
        for (int i = 2; i <= target; i++) {
            y = 2 * x;
            x = y;
        }
        return y;
    }
}