LeetCode-Implement strStr()

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：实现字符串函数strStr()的功能，返回字符串haystack第一次出现字符串needle的位置，如果没有则返回-1；对于needle字符串为空的情况则返回0；

解法：只要遍历字符串haystack，每次计算其长度为needle.length()的字串是否和needle相等即可；

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