递归计算以 sub 开头和结尾的最大子串并返回其长度 [英] Compute recursively the largest substring which starts and ends with sub and return its length
问题描述
任务是:给定一个字符串和一个非空子串 sub,递归计算以 sub 开头和结尾的最大子串并返回其长度.
The task is: Given a string and a non-empty substring sub, compute recursively the largest substring which starts and ends with sub and return its length.
示例:
strDist("catcowcat", "cat") → 9
strDist("catcowcat", "cow") → 3
strDist("cccatcowcatxx", "cat") → 9
你能看看我的代码并告诉我它有什么问题吗?
Can you please look at my code and tell me what is the problem with it?
public int strDist(String str, String sub)
{
if(str.length()<sub.length())
return 0;
if(str.length()==sub.length()&&str.equals(sub))
return str.length();
if(str.length()<2)
{
if(str.contains(sub))
{
return 1;
}
return 0;
}
if (str.length()==2)
{
if (sub.length()==2 && str.equals(sub))
return 2;
if (str.contains(sub))
return 1;
return 0;
}
if(str.length()>2)
{
if(str.startsWith(sub)&&str.endsWith(sub))
{
return str.length();
}
if(str.substring(0,sub.length()).equals(sub))
{
strDist(str.substring(0,str.length()-2),sub);
}
if(str.substring(str.length()-sub.length(),str.length()-1).equals(sub))
strDist(str.substring(1,str.length()-1),sub);
}
return strDist(str.substring(1,str.length()-1),sub);
}
它不适用于这种情况 strDist("hiHellohihihi", "hih")
→ 5并返回零.
it doesn't work for the case strDist("hiHellohihihi", "hih")
→ 5
and returns zero.
推荐答案
首先,为了回答您的问题,我在您的代码中发现了许多问题.下面是我更正的版本,并附有对我所做更改的评论.
First, to answer your question, I found a number of issues in your code. My corrected version follows, with comments about the changes I did.
public int strDist(String str, String sub) {
if (str.length() < sub.length())
return 0;
// simplified condition
if (str.equals(sub))
return str.length();
if (str.length() < 2) {
if (str.contains(sub)) {
// corrected (if str and sub are both empty strings, you don’t want to return 1)
return str.length();
}
return 0;
}
// deleted str.length() == 2 case that didn’t work correctly
if (str.startsWith(sub) && str.endsWith(sub)) {
return str.length();
}
if (str.startsWith(sub)) { // simplified
// subtracting only 1 and added return statement
return strDist(str.substring(0, str.length() - 1), sub);
}
// changed completely -- didn’t understand; added return statement, I believe this solved your test case
if (str.endsWith(sub))
return strDist(str.substring(1), sub);
return strDist(str.substring(1, str.length() - 1), sub);
}
现在如果我这样做:
System.out.println(strDist("catcowcat", "cat"));
System.out.println(strDist("catcowcat", "cow"));
System.out.println(strDist("cccatcowcatxx", "cat"));
System.out.println(strDist("hiHellohihihi", "hih"));
我明白了:
9
3
9
5
第二,正如我在评论中所说,我认为在这里使用递归没有意义(除了练习).您的方法的以下版本没有,它更简单并且工作原理相同:
Second, as I said in a comment, I see no point in using recursion here (except perhaps for the exercise). The following version of your method doesn’t, it’s much simpler and it works the same:
public int strDist(String str, String sub) {
int firstOccurrence = str.indexOf(sub);
if (firstOccurrence == -1) { // sub not in str
return 0;
}
int lastOccurrence = str.lastIndexOf(sub);
return lastOccurrence - firstOccurrence + sub.length();
}
最后,这可能有用也可能没用,递归版本不需要像你的那么复杂:
Finally, and this may or may not be helpful, a recursive version needs not be as complicated as yours:
public int strDist(String str, String sub) {
if (sub.isEmpty()) {
throw new IllegalArgumentException("sub mustn’t be empty");
}
if (str.length() <= sub.length()) {
if (str.equals(sub)) {
return str.length();
} else { // sub cannot be in str
return 0;
}
}
if (str.startsWith(sub)) {
if (str.endsWith(sub)) {
return str.length();
} else {
return strDist(str.substring(0, str.length() - 1), sub);
}
} else {
return strDist(str.substring(1), sub);
}
}
如果可以的话,最好先让一些东西开始工作,即使这不是最简单和优雅的解决方案.当它有效或无效时,是考虑简化方法的好时机.这样可以更容易地确定错误,也可以方便以后的维护.特殊情况,如长度 1 和长度 2,通常是简化的一个很好的候选:看看通用代码是否已经满足它们或者可以很容易地实现.
It’s fine to get something to work first if you can, even if it’s not the most simple and elegant solution. When either it works or it doesn’t, is a good time to think of ways to simplify. It will make it easier to nail down the bug(s) and also ease maintenance later. Special cases, like length 1 and length 2, are often a good candidate for simplification: see if the general code already caters for them or can easily be made to.
这篇关于递归计算以 sub 开头和结尾的最大子串并返回其长度的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持IT屋!