模式匹配——BF算法和KMP算法

理论知识参考来自：

KMP 算法详解 - 知乎 (zhihu.com)

具体代码可以参考：

#include<iostream>
#include<vector>
using namespace std;

int BF_Search(string& txt, string &pat) {
	int M = txt.length();
	int N = pat.length();
	for (int i = 0; i <= M - N; i++) {
		int j;
		for (j = 0; j < N; j++) {
			if (pat[j] != txt[i + j])
				break;
		}
		if (j == N)
			return i;
	}
	return -1;
}

class KMP_algorithm
{
private:
	vector<vector<int>> dp;
	string pat;
	
	void DP_Calculate() {
		int X = 0;
		int size = this->pat.length();
		dp[0][pat[0]] = 1;
		for (int j = 1; j < size; j++) {
			for (int c = 0; c < 256; c++)
			{
				if (pat[j] == c)
					dp[j][c] = j + 1;
				else
					dp[j][c] = dp[X][c];
			}
			X = dp[X][pat[j]];
		}
	}

public:
	KMP_algorithm(string pat) {
		this->pat = pat;
		int row = pat.length();
		vector<int> temp(256);
		dp.resize(row, temp);
		this->DP_Calculate();
	}
	int KMP_Searh(string txt) {
		int M = txt.length();
		int N = pat.length();
		int j = 0;
		for (int i = 0; i < M; i++) {
			j = dp[j][txt[i]];
			if (j == N)
				return i - N + 1;
		}
		return -1;
	}


};

int main() {
	string txt = "ababc";
	string tex = "abcabc";
	string pat = "abc";
	KMP_algorithm str(pat);
	cout<<str.KMP_Searh(txt)<<endl;//2
	cout<<str.KMP_Searh(tex) << endl;//0
	system("pause");
	return 0;
}