C++的KMP算法

KMP算法作用：字符串匹配

例如母串S = “aaagoogleaaa”;

子串T= “google”;

步骤1：先计算子串中的前后缀数组Next

C++代码：

//步骤1：
void GetNext(string Tsub, vector<int>& Next)
{
int j = 0, k = -1;
Next[0] = k;
while (j < Tsub.length() - 1)
{
if (k == -1 || Tsub[j] == Tsub[k])
{
Next[++j] = ++k;
}
else
{
k = Next[k];
}
}
}

步骤2：查找子串在母串中出现的位置。

//步骤2：
int KMP(string S, string T, vector<int> Next)
{
int i = 0, j = 0;
int m = S.length();
int n = T.length();
while (i < m && j < n)
{
if (j == -1 || S[i] == T[j])
{
i++;
j++;
}
else
{
j = Next[j];
}
}
if (j == n)
{
return i - j;
}
else
{
return -1;
}
}

结合上面两个步骤写出完整代码：

#include <iostream>
#include <vector>
using namespace std;
//步骤1：
void GetNext(string Tsub, vector<int>& Next)
{
int j = 0, k = -1;
Next[0] = k;
while (j < Tsub.length() - 1)
{
if (k == -1 || Tsub[j] == Tsub[k])
{
Next[++j] = ++k;
}
else
{
k = Next[k];
}
}
}
//步骤2：
int KMP(string S, string T, vector<int> Next)
{
int i = 0, j = 0;
int m = S.length();
int n = T.length();
while (i < m && j < n)
{
if (j == -1 || S[i] == T[j])
{
i++;
j++;
}
else
{
j = Next[j];
}
}
if (j == n)
{
return i - j;
}
else
{
return -1;
}
}
int main()
{
string S = "aaagoogleaaa";
string T = "google";
vector<int> Next(T.length());
GetNext(T, Next);
int retVal = KMP(S, T, Next);
if (retVal == -1)
{
std::cout << "can't Index" << std::endl;
}
else
{
std::cout << "Index :" << retVal << std::endl;
}
return 0;
}