[LeetCode]KMP——字符串匹配

一、问题描述：
https://leetcode.com/problems/implement-strstr/
Implement strStr().

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

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二、实现：
暴力匹配

int strStr(string haystack, string needle) {
if (haystack.length() < needle.length()) {
return -1;
}
if (haystack.length() == needle.length()) {
if (haystack == needle) {
return 0;
}
return -1;
}
for (int i = 0; i < haystack.length()-needle.length()+1; ++i) {
string tmp = haystack.substr(i, needle.length());
cout << tmp << endl;
if (tmp == needle) {
return i;
}
}
return -1;
}

KMP
注意大坑！

j<(int)needle.length()

不加(int)的话，由于length()为无符号类型，当j==-1时将会被转化为INT_MAX！

public:
int strStr(string haystack, string needle) {
if (needle.empty()) return 0;
if (haystack.empty()) return -1;
if (haystack.length() == needle.length()) {
if (haystack == needle) return 0;
return -1;
}
vector<int> nextArray(needle.length(), 0);
generateNext(nextArray, needle);
int i = 0, j =0;
while(i<haystack.length() && j<(int)needle.length()) {
if (j==-1 || haystack[i]==needle[j]) {
i++;
j++;
}
else j = nextArray[j];
}
if (j == needle.length()) return i-j;
return -1;
}
private:
void generateNext(vector<int> &nextArray, string needle) {
nextArray[0] = -1;
int k = -1, j = 0;
while(j < needle.length()-1) {
if (k==-1 || needle[k]==needle[j]) {
k++;
j++;
if (needle[j] != needle[k]) nextArray[j] = k;
else nextArray[j] = nextArray[k];
}
else k = nextArray[k];
}
}

附：KMP讲解 http://blog.csdn.net/v_july_v/article/details/7041827