28. Implement strStr()
Tags: ‘Two Pointers’, ‘String’
Implement strStr().
Return the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.
Example 1:
Input: haystack = "hello", needle = "ll" Output: 2
Example 2:
Input: haystack = "aaaaa", needle = "bba" Output: -1
Clarification:
What should we return when needle is an empty string? This is a great question to ask during an interview.
For the purpose of this problem, we will return 0 when needle is an empty string. This is consistent to C's strstr() and Java's indexOf().
Solution
方法1 Naive $O(M(N-M)) O(1)$
public int strStr(String haystack, String needle) {
// Naive
if (haystack == null || needle == null) return -1;
int N = haystack.length();
int M = needle.length();
if (M == 0) return 0;
for (int i = 0; i <= N - M; i++) {
int j;
for (j = 0; j < M; j++) {
if (haystack.charAt(i + j) != needle.charAt(j)) {
break;
}
}
if (j == M) {
return i;
}
}
return -1;
}
方法2 KMP (Knuth Morris Pratt) Pattern Searching $O(n)$ 考到的概率很小