Implement strStr()

题目地址：
https://leetcode.com/problems/implement-strstr/#/description

题目：

Implement strStr().

Returns the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.

解题思路：
常规思路就是先找到第一个相同的char，然后y比较之后的每个char。

代码：

public int strStr(String haystack, String needle) {
    if(haystack == null || needle == null){
        return -1;
    }
    int len1 = haystack.length();
    int len2 = needle.length();
    for(int i = 0; i <= len1 - len2; i++){
        int j = 0;
        while(j <= len2 - 1 && haystack.charAt(i + j) == needle.charAt(j)){
            j++;
        }
        if(j == len2){
            return i;
        }
    }
    return -1;
}