Description：
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().

题意：模拟strStr()函数的功能，找出字符串str1在字符串str中首次出现的位置，返回首个字符的下标；

解法：最简单的办法就是从串的左侧一直匹配到串的右侧，直到找到匹配项或者遍历完一次字符串；

class Solution {
    public int strStr(String haystack, String needle) {
        if(needle == null){
            return 0;
        }
        int st,ed;
        st = 0;
        ed = needle.length();
        while(ed <= haystack.length()){
            if(haystack.substring(st, ed).equals(needle)){
                return st;
            }
            st++;
            ed++;
        }
        return -1;
    }
}