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028_Implement_strStr().cpp
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028_Implement_strStr().cpp
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/*
Implement strStr().
Returns the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.
*/
// Straightforward Solution: Time O(mn)
class Solution {
public:
int strStr(string haystack, string needle) {
if (haystack.size() < needle.size()) return -1;
if (needle.empty()) return 0;
int m = haystack.size();
int n = needle.size();
for (int i = 0; i <= m-n; i++) {
for (int j = 0; j <= n; j++) {
if (j == n) return i;
if (haystack[i+j] != needle[j]) break;
}
}
return -1;
}
};
// KMP solution: Time = O(m+n); Space = O(n).
class Solution {
public:
int strStr(string haystack, string needle) {
if (haystack.size() < needle.size()) return -1;
if (needle.empty()) return 0;
int m = haystack.size();
int n = needle.size();
vector<int> next(n, -1);
nextTable(needle, next);
int j = -1;
for (int i = 0; i < m; i++) {
while (j >= 0 && haystack[i] != needle[j+1]) j = next[j];
if (haystack[i] == needle[j+1]) j++;
if (j == n-1) return i-n+1;
}
return -1;
}
void nextTable(string needle, vector<int> &next) {
int j = -1;
for (int i = 1; i < needle.size(); i++) {
while (j >= 0 && needle[i] != needle[j+1]) j = next[j];
if (needle[i] == needle[j+1]) j++;
next[i] = j;
}
}
};