题目描述：
Implement strStr().
Returns the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.
分析：
这个题目考察的是KMP算法，这个算法的核心是求出子串的自匹配数组，在失配情况下不回溯父串指针（其实是索引），只需将子串向左滑动即可，next数组就是记录了滑动的距离的数组，求next数组是这个算法的难点。
我的第一个版本的代码如下：
#include <iostream>
#include <string>
#include <vector> using namespace std; int strStr(string haystack, string needle);
void getNext(vector<int> &ivec, const string & s); int main()
{
cout << "Hello world!" << endl;
string hay("mississippi");
string needle("issi");
int res = strStr(hay, needle);
cout << "the result of matching:" << res << endl;
system("pause");
return ;
} void getNext(vector<int> &ivec, const string & s)
{
int i = , j = -;
ivec[i] = j;
while (i < s.size() - )
{
if (j == - || s[i] == s[j])
ivec[++i] = ++j;
else
j = ivec[j];
}
} int strStr(string haystack, string needle)
{
if (needle.size() == )
return ;
if (haystack.size() == )
return -; vector<int> ivec(needle.size(), );
getNext(ivec, needle); for (auto i = ivec.begin(); i != ivec.end(); i++)
cout << *i << endl; int i = , j = ;
cout << haystack.size() << " " << needle.size() << endl; while( i < haystack.size() && j < needle.size() )
//while (i != haystack.size() && j != needle.size())
{
cout << "before modified " << i << " " << j << endl;
if (j == - || haystack[i] == needle[j])
{
cout << "what" << endl;
++i;
++j;
}
else
{
j = ivec[j];
}
cout << "after modified " << i << " " << j << endl; }
if (j == needle.size())
return i - j;
else
return -; }
这份代码我以为可以完美AC了，结果报了wrong answer，我很不理解，经过修改，就是将上一个版本中的while循环头换成下方紧邻的那一行就AC了，经过和大神的讨论，大神很快指出了我的错误所在，那就是C++的隐式类型转换！haystack.size()返回的是一个unsigned int类型，而i， j是int型，在运算时int会隐式转换为unsigned int，而-1转换为unsigned int型之后为4294967295（2的32次方减1）是一个非常大的数字！以前写循环不注意，现在吃了大亏！希望吃一堑长一智吧，以后写代码一定要非常注意类型的选择，注意int和unsigned int不能比较，如果必须比较一定将unsigned int 强制转换为long long类型再比较！最后完成版的代码如下：
#include <iostream>
#include <string>
#include <vector> using namespace std; int strStr(string haystack, string needle);
void getNext(vector<int> &ivec, const string & s); int main()
{
string hay("mississippi");
string needle("issi");
int res = strStr(hay, needle);
cout << "the result of matching:" << res << endl;
system("pause");
return ;
} void getNext(vector<int> &ivec, const string & s)
{
int i = , j = -;
ivec[i] = j;
while (i < s.size() - )
{
if (j == - || s[i] == s[j])
ivec[++i] = ++j;
else
j = ivec[j];
}
} int strStr(string haystack, string needle)
{
if (needle.size() == )
return ;
if (haystack.size() == )
return -; vector<int> ivec(needle.size(), );
getNext(ivec, needle); for (auto i = ivec.begin(); i != ivec.end(); i++)
cout << *i << endl; int i = , j = ;
long long haystack_size = haystack.size();
long long needle_size = needle.size(); while( i < haystack_size && j < needle_size )
{
cout << "i and j before modified " << i << " " << j << endl;
if (j == - || haystack[i] == needle[j])
{
cout << "what" << endl;
++i;
++j;
}
else
{
j = ivec[j];
}
cout << "i and j after modified " << i << " " << j << endl;
haystack_size = haystack.size();
needle_size = needle.size(); }
if (j == needle.size())
return i - j;
else
return -; }