蓝桥杯之KMP算法

算法思想

代码实现

int* getnext()
{
	int* next = new int[s2.size()];
	int j = 0;//用来遍历子串
	int k = -1;//子串中公共子串的长度
	next[0] = -1;
	while (j < s2.size() - 1)
	{
		if (k==-1||s2[k] == s2[j])
		{
			k++;
			j++;
			if (s2[k] == s2[j])
			{
				next[j] = next[k];
			}
			else
			{
				next[j] = k;
			}
		}
		else
		{
			k = next[k];//做k值得回溯，继续找最长公共前后缀
		}
	}
	return next;
}
int KMP()
{
	{
		int i = 0;
		int j = 0;
		//是为了处理当j==-1时的情况
		int len1 = s1.size();
		int len2 = s2.size();
		int* next = getnext();
		while (i < len1 && j < len2)
		{
			if (j==-1||s1[i] == s2[j])
			{
				i++;
				j++;
			}
			else
			{
				j = next[j];
			}
		}
		if (j == s2.size())
		{
			return i - j;
		}
		else
		{
			return -1;
		}
	}
}
int main()
{
	int pos = KMP();
	cout << pos << endl;
	return 0;
}