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分享[面试题求解] 两道递归选择题求解
两道都是不定项选择题
10. The recursive function mystrlen(char *buf, int N)defined below tries to find the length of the first null-terminated string in the buffer buf(not counting the null character), where the buffer size is N. For instance if
buf = {'b','u','f','f','e','r','\0','a','b','c'}
with N = 10 is the input, the desired output is 6. If the buffer does not have any null character the desired output is N.
int mystrlen(char *buf, int N)
{
return mystrlen(buf, N/2) + mystrlen(buf + N/2, N/2);
}
What are all the possible mistakes in the code? （代码中可能的错误是哪个/些）
A. There are no mistakes in the code
B. There is no termination of recursion
C. The addition of the the two mystrlen()s in the recursion is incorrect
D. The use of N/2 in the recursion is incorrect
E. Recursion cannot be used to calculate this function
11. Continuing the above example, which of the following recursive implementations fix the code fully? （如果要修正以上的错误，应该）
A. No change to the code in example above
B.
int mystrlen(char *buf int N)
{
if(N==0)
return 0;
else
return mystrlen(buf, N/2) + mystrlen(buf + N/2, N/2);
}
C.
int mystrlen(char *buf int N)
{
if(N==0||buf[0]==0)
return 0;
else if (N==1)
return 1;
int t = mystrlen(buf, N/2);
if(t<N/2)
return t;
else
return (t + mystrlen(buf + N/2, (N+1)/2));
}
D. None of above
10. The recursive function mystrlen(char *buf, int N)defined below tries to find the length of the first null-terminated string in the buffer buf(not counting the null character), where the buffer size is N. For instance if
buf = {'b','u','f','f','e','r','\0','a','b','c'}
with N = 10 is the input, the desired output is 6. If the buffer does not have any null character the desired output is N.
int mystrlen(char *buf, int N)
{
return mystrlen(buf, N/2) + mystrlen(buf + N/2, N/2);
}
What are all the possible mistakes in the code? （代码中可能的错误是哪个/些）
A. There are no mistakes in the code
B. There is no termination of recursion
C. The addition of the the two mystrlen()s in the recursion is incorrect
D. The use of N/2 in the recursion is incorrect
E. Recursion cannot be used to calculate this function
11. Continuing the above example, which of the following recursive implementations fix the code fully? （如果要修正以上的错误，应该）
A. No change to the code in example above
B.
int mystrlen(char *buf int N)
{
if(N==0)
return 0;
else
return mystrlen(buf, N/2) + mystrlen(buf + N/2, N/2);
}
C.
int mystrlen(char *buf int N)
{
if(N==0||buf[0]==0)
return 0;
else if (N==1)
return 1;
int t = mystrlen(buf, N/2);
if(t<N/2)
return t;
else
return (t + mystrlen(buf + N/2, (N+1)/2));
}
D. None of above
...全文
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fsy351 2013-03-05
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貌似组合数学或数论的东西啊。。。
onlyonename 2013-03-04
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0 1 1 2 3 5 8 3 1 4 5 9 4 3 7 0 7 0 7 0 .....
看明白？
skywalker0725 2013-03-02
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我晕，都没看懂~~~
d529684722 2013-03-01
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hao nan
林初茵 2013-02-27
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谢谢大家 基本明白了
赵4老师 2013-02-27
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“给定一个小点的输入，完整单步跟踪(同时按Alt+7键查看Call Stack里面从上到下列出的对应从里层到外层的函数调用历史)一遍。”是理解递归函数工作原理的不二法门！
递归函数关注以下几个因素
·退出条件
·参数有哪些
·返回值是什么
·局部变量有哪些
·全局变量有哪些
·何时输出
·会不会导致堆栈溢出
雷神风暴烈酒 2013-02-26
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楼主
http://bbs.csdn.net/topics/390372779?page=1#post-393768505
去看看我的回复 不知是否正确
yiuwing 2013-02-26
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1 BC
2 C
ken_scott 2013-02-26
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其他两个看不懂
ken_scott 2013-02-26
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记:
result = F(n) % 5 (1)
可推导出:
F(n) = 5 * F(n-4) + 3 * F(n-5)
得:
result = F(n) % 5 = (3 * F(n-5)) % 5 (2)
由(1), (2)可知:
在此问题中, F(n) 与 3 * F(n-5) 等效,
记做 F(n) <==> 3 * F(n-5) (3)
于是递归可得:
F(n-5) <==> 3 * F((n-5) - 5)
即有:
3 * F(n-5) <==> 3 * ( 3 * F((n-5) - 5)
<==> (9 * F(n-10))
<==> (5 * F(n-10) + 4 * F(n-10))
<==> (4 * F(n-10)) (4)
由(3), (4)可知:
F(n) <==> 4 * F(n-10) (5)
于是可得:
4 * F(n-10) <==> 4 * (4 * F((n-10) - 10)
<==> 16 * F(n-20)
<==> 15 * F(n-20) + 1 * F(n-20)
<==> F(n-20) (6)
由(5), (6)可知:
F(n) <==> F(n-20)
所以, F(1025) <==> F(1005) <==> F(985) <==> F(965) <==> F(5)
由(3)知: F(5) <==> 3 * F(5-5) <==> 3 * F(0) <==> 0
所以, F(1025) % 5 <==> 0 % 5 <==> 0
即有: F(1025) % 5 == 0
maoloverme1 2013-02-26
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如果null出现在mystrlen(buf, N/2)中，那么就不能加上mystrlen(buf + N/2, N/2);
林初茵 2013-02-26
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还是不懂 木乱阿
雷神风暴烈酒 2013-02-26
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return mystrlen(buf, N/2) + mystrlen(buf + N/2, N/2);
并不能得到所有答案
林初茵 2013-02-26
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1 b我能理解 c为什么也选了阿？ 求解是
雷神风暴烈酒 2013-02-26
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确实。。应该是BC
雷神风暴烈酒 2013-02-26
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10 B 11 C 可对否
maoloverme1 2013-02-26
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1 BC
2 C
林初茵 2013-02-26
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谢谢 那其他两道题呢？
雷神风暴烈酒 2013-02-26
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正解
IT民工TI 2013-02-26
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看了下你们的回复,这样应该是好懂点吧
f(5n) = f(5n-1) + f(5n-2)
= f(5n-2) + 2f(5n-3) + f(5n-4)
= 5*f(5n-4) + 3*f(5(n-1));
3*f(5(n-1))除5取余的值与 f(5n)一致
以此类推，f(5n)%5 与 f(5) 一致，所以为0
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