Leetcode28. 找出字符串中第一个匹配项的下标

代码：

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