28. Implement strStr() [LeetCode]

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28. Implement strStr()

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().

class Solution {
public:
	int strStr(string haystack, string needle) {
		if (needle.empty())return 0;
		int m = haystack.size(), n = needle.size();
		if (m < n)return -1;

		for (int i = 0; i <= m - n; ++i) {
			int j = 0;
			for (j = 0; j < n; ++j)
				if (needle[j] != haystack[i + j]) 
					break;			
			if (j == n) return i;
		}
		return -1;
	}
};