[leetcode] 21. Implement strStr()

这个题目是典型的KMP算法，当然也可以试试BM，当然有关KMP和BM的介绍阮一峰曾经写过比较好的科普，然后july也有讲解，不过那个太长了。

先放题目吧：

Implement strStr().

Returns the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.

Update (2014-11-02):

The signature of the function had been updated to return the index instead of the pointer. If you still see your function signature returns a char * or String, please click the reload button to reset your code definition.

就是找到haystack内是否有needle这个字符串，字符串查找算法。

KMP主要是建立一个next的部分匹配数组，来用空间换时间。next的生成函数是KMP关键，解法如下：

class Solution {
public:
	vector<int> generateNext(char *str)
	{
		int len = strlen(str);
		vector<int> tmp;
		tmp.push_back(0);

		for (int i = 1, j = 0; i < len; i++)
		{
			while (j > 0 && str[i] != str[j])
			{
				j = tmp.at(j - 1);
			}

			if (str[i] == str[j])
			{
				j++;
			}

			tmp.push_back(j);
		}

		return tmp;
	}

	int strStr(char *haystack, char *needle) 
	{
		vector<int> next = generateNext(needle);

		int lenH = strlen(haystack);
		int lenN = strlen(needle);

		if (lenN == 0)
		{
			return 0;
		}
		if (lenH < lenN)
		{
			return -1;
		}
		int i, j;

		for (i = 0, j = 0; i < lenH; i++)
		{
			while (j > 0 && haystack[i] != needle[j])
			{
				j = next[j - 1];
			}

			if (haystack[i] == needle[j])
			{
				j++;
			}

			if (j == lenN)
			{
				return i - lenN + 1;
			}
		}
		return -1;
	}
};