青蛙跳台阶，类似于Fibonacci

public class Solution {
//解法一
public int jumpFloor(int target) {
if( target == 1 || target == 2){
return target;
}
return jumpFloor(target-1) + jumpFloor(target-2);
}

//解法二
public int jumpFloor(int target){
    if(target == 1 || target == 2){
        return target;
    }
    int ans[] = new int[target];
    ans[0] = 1;
    ans[1] = 2;
    for(int i = 2;i <= target-1;i++){
        ans[i] = ans[i-1] + ans[i-2];
    }
    return ans[target - 1];
}
//解法三
public int jumpFloor(int target){
    if(target == 1 || target == 2){
        return target;
    }
    int temp = 1;
    int ans = 2;
    for(int i = 2;i <= target-1;i++){
        ans = ans + temp;
        temp = ans - temp;
    }
    return ans;
}
//解法四
public int jumpFloor(int target) {//从1开始，target+1;从0开始，target
    double a = Math.sqrt(5.0);
    double temp1 = (1.0 + a)/2.0;
    double temp2 = (1.0 - a)/2.0;
    return (int)(1.0/a*(Math.pow(temp1, target+1) - Math.pow(temp2, target+1)));

}

}