KMP算法

#include 
#include 
#include 

using namespace std;

class KMP {
public:
	KMP();
	~KMP();
	int getNext(string subStr);
	int getIndex(string mainStr,string subStr);
private:
	vector<int> next;
};

int main(){
	string mainStr;
	string subStr;
	cout << "KMP algorithm test:" << endl;
	cout << "please input main string:" << endl;
	getline(cin,mainStr);
	cout << "please input substring:" << endl;
	cin >> subStr;
	KMP kmpIndex;
	cout << "result is :  " << kmpIndex.getIndex(mainStr, subStr) << endl;
	return 0;
}

KMP::KMP(){
}

KMP::~KMP(){
}

int KMP::getNext(string subStr){
	int k = -1;
	next.push_back(-1);
	for (int j = 0; j < subStr.length() - 1;) {
		if (k == -1 || subStr[j] == subStr[k]) {
			j++;
			k++;
			next.push_back(k);
		}
		else {
			k = next[k];
		}
	}
	return 0;
}

int KMP::getIndex(string mainStr,string subStr){
	int i = 0;
	int j = 0;
	int mainStrLength = mainStr.length();//注1
	int subStrLength = subStr.length();
	getNext(subStr);
	while(i < mainStrLength&&j < subStrLength){
		
		if (j == -1 || mainStr[i] == subStr[j]) {
			i++; 
			j++;
		}
		else {
			j = next[j];
		}
	}
	if (j >= subStr.length()) {
		return (i - subStr.length());
	}
	else {
		return -1;
	}
}

注：1.在C++中string的length()返回的字符串长度的类型是unsigned int，在int KMP::getIndex(string mainStr,string subStr)函数里，会有j=-1的情况发生，这时就会出现错误，因此需要对数据类型进行处理。