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;
}
}
