练习题

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上台阶
有一楼梯共m级，刚开始时你在第一级，若每次只能跨上一级或者二级，要走上m级，共有多少走法？注：规定从一级到一级有0种走法。
给定一个正整数int n，请返回一个数，代表上楼的方式数。保证n小于等于100。为了防止溢出，请返回结果Mod 1000000007的值。
测试样例：
3
返回：2
Java代码

package com.algorithm;/** * Created by wwh on 2017/12/8. */public class Main01 {    public static  void main(String argc[]) {        GoUpstairs obj = new GoUpstairs();        int result = obj.countWays(50);        System.out.println(result);    }    public static class GoUpstairs {        public int countWays(int n) {            final int VALUE = 1000000007;            if(n == 1) return 0;            else if(n == 2) return 1;            else if(n == 3) return 2;            int res;            int  first = 1, second = 2, ret = 0;            while(n > 3) {                ret = (first + second)%VALUE;                first = second;                second = ret;                n--;            }            return ret;        }    }}

python代码

class GoUpstairs:    def countWays(self, n):        res = [0, 1, 2]        while len(res) < n:            res.append(res[-1] + res[-2])        return res[n-1]%1000000007obj = GoUpstairs()res = obj.countWays(6)print res

class GoUpstairs:    def countWays(self, n):        if n < 1:            return 0        res = [0, 1, 2]        if n > 3:            for i in range(3, n):                res.append((res[i - 1] + res[i - 2])%1000000007)        return res[n - 1]if __name__ == "__main__":    maxNumStairs = 100    print("num stairs: (Results)")    for cnt in range(1, maxNumStairs):        stairs = GoUpstairs()        res = stairs.countWays(cnt)        print("%d stairs: (%d)" % (cnt, res))

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