Leetcode 28. Implement strStr()

https://leetcode.com/problems/implement-strstr/

class Solution {
public:
void KMP(string needle, vector<int> &nextval) {
	nextval[0] = -1;//1长度没有真前后缀的公共部分
	int x = -1;
	for (int k = 1; k < needle.size(); ++k) {
		//要等needle[0...k-1]和needle[0...x]匹配才行【x表示needle[0...k-2]的从大到小的真前后缀公共部分】
		while (x!=-1 && needle[k-1] != needle[x]) { 
			x = nextval[x];
		}
		++x;
		if (needle[x] == needle[k]) {//考虑是匹配needle[k]遇到的失配
			nextval[k] = nextval[x];
		}
		else {
			nextval[k] = x;
		}
	}

}
int strStr(string haystack, string needle) {
	if (needle.empty()) return 0;
	if (haystack.empty()) return -1;
	/*
	KMP算法
	*/
	vector<int> nextval(needle.size(), 0);
	KMP(needle, nextval);
	int x = 0;
	for (int k = 0; k < haystack.size(); ++k) {

		while (x != -1 && haystack[k] != needle[x]) {
			x = nextval[x];
		}

		if (++x == needle.size()) return k + 1 - needle.size();

	}
	return -1;
}
};