从第一个字符开始遍历，符合则返回位置即可

class Solution {
    public int strStr(String haystack, String needle) {
        if (needle.length() == 0) return 0;
        if (haystack.length() == 0) return -1;
        for (int i=0;i<haystack.length()-needle.length()+1;i++)
            if (isMatch(haystack,i,needle))
                return i;
        return -1;
    }
    private boolean isMatch(String haystack, int index, String needle){
        for (int i=0;i<needle.length();i++){
            if (haystack.charAt(index+i) != needle.charAt(i)) 
                return false;
        }
        return true;
    }
}