KMP算法

具体知识参考：http://blog.youkuaiyun.com/v_july_v/article/details/7041827

1、计算模式的next数组

2、使用next辅助进行匹配

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

vector<int> GetNext(string &p)
{
	int len = p.size();
	vector<int> next(len);
	next[0] = -1;
	int k = -1;  
	int j = 1;  //j是next数组的下标
	while (j<len)
	{
		if (k == -1 || p[k] == p[j])
		{
			k++;       //
			next[j] = k;
			j++;      //更新next的下一位置
		}
		else
		{
			k = next[k];
		}
	}
	return next;
}

int KMP(string &s, string &p)
{
	vector<int> next = GetNext(p);
	int len1 = s.size();
	int len2 = p.size();
	int i = 0;
	int j = 0;
	while (i < len1 && j < len2)
	{
		if (j==-1 || s[i] == p[j])
		{
			i++;
			j++;
		}
		else
		{
			j = next[j];
		}
	}
	if (j == len2) return i - j;
	else return -1;
}

int main()
{
	string s = "abcabsdac";
	string p = "absd";
	cout << KMP(s,p);
	cout << endl;
	system("pause");
	return 0;
}