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// 28. Find the Index of the First Occurrence in a String
#include "leetcode.h"
#include <vector>
/*
* given two strings 'needle' and 'haystack', return index of the first
* occurrence of 'needle' in 'haystack', or -1 if 'needle' is not part of
* 'haystack'
*/
class Solution {
public:
int strStr(string haystack, string needle) {
int m = haystack.size(), n = needle.size();
if (!n)
return 0;
vector<int> lps = kmp(needle);
for (int i = 0, j = 0; i < m;) {
if (haystack[i] == needle[j])
i++, j++;
if (j == n)
return i - j;
if (i < m && haystack[i] != needle[j])
j ? j = lps[j - 1] : i++;
}
return -1;
}
private:
// kmp algorithm:
// http://jakeboxer.com/blog/2009/12/13/the-knuth-morris-pratt-algorithm-in-my-own-words/
vector<int> kmp(string needle) {
int n = needle.size();
vector<int> lps(n, 0);
for (int i = 1, len = 0; i < n;) {
if (needle[i] == needle[len])
lps[i++] = ++len;
else if (len)
len = lps[len - 1];
else
lps[i++] = 0;
}
return lps;
}
};
int main() {
Solution obj;
cout << obj.strStr("sadbutsad", "sad") << endl; // expect: 0
cout << obj.strStr("leetcode", "leeto") << endl; // expect: -1
}
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