[leetcode] Implement strStr()

题目链接在此

Implement strStr().

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

模式匹配。KMP咯。说起KMP的话，自从大一上学C语言开始就有所了解，但是只知道它的用途和大概原理，从没下过功夫落实到具体的代码上。写这篇东西，依然只是把别人的想法和代码记录下来。



· KMP算法详解

· 代码与优化方法

class Solution {
public:
int strStr(string haystack, string needle) {
if (haystack.empty())
return needle.empty() ? 0 : -1;

if (needle.empty())
return 0;

int i = 0, j = 0;
vector<int> next(needle.length() + 1);
getNext(next, needle);

while (i != haystack.length()) {
while (j != -1 && haystack[i] != needle[j])
j = next[j];
++i; ++j;
if (j == needle.length()) return i - j;
}
return -1;
}

private:
void getNext(vector<int> &next, string &needle) {
int i = 0, j = -1;
next[i] = j;
while (i != needle.length()) {
while (j != -1 && needle[i] != needle[j])
j = next[j];
next[++i] = ++j;
}
}
};