Implement strStr()

题目描述：

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()]

分析：判断字符串needle是否存在于haystack字符串中，如果存在返回needle在haystack字符串中的起始下标index，否则返回-1；后面的声明中提示，如果needle为空时，返回0；

我的Code如下：

class Solution {
    public int strStr(String haystack, String needle) {
        int index = -1;
        if(needle.length() == 0){
            return 0;
        }
        
        for(int i = 0; i<=haystack.length()-needle.length();i++){
            if(haystack.substring(i,i+needle.length()).equals(needle)){
                return i;
            }
        }
        return index;
    }
}