LeetCode28 Implement strStr()

题目描述：
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().
题源：here；完整代码：here
思路：
遍历haystack，如果有与needle相同的情况就看haystack和needle之后的是否相同。
代码

    int strStr(string haystack, string needle) {
        if (needle.size() == 0) return 0;
        if (haystack.size() == 0) return -1;

        for (int i = 0; i < haystack.size(); i++){
            int needle_idx = 0;
            int haystack_idx = i;
            while (haystack_idx < haystack.size() &&\
                   needle_idx < needle.size()     &&\
                   haystack[haystack_idx] == needle[needle_idx])
            {
                needle_idx++;
                haystack_idx++;
            }

            if (needle_idx == needle.size()) return i;
        }

        return -1;
    }