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().

难度: easy

题目：
实现方法strstr()
返回首次needle出现在haystack中的索引，如果不存在则返回 -1.

Runtime: 7 ms, faster than 37.21% of Java online submissions for Implement strStr().
Memory Usage: 26.9 MB, less than 32.93% of Java online submissions for Implement strStr().

class Solution {
    public int strStr(String haystack, String needle) {
        if (needle.length() > haystack.length()) {
            return -1;
        }
        
        for (int i = 0; i <= haystack.length() - needle.length(); i++) {
            int j = 0;
            for (; j < needle.length(); j++) {
                if (haystack.charAt(i + j) != needle.charAt(j)) {
                    break;
                }
            }
            if (j == needle.length()) {
                return i;
            }
        }
        
        return -1;
    }
}