原题链接: https://oj.leetcode.com/problems/implement-strstr/

public int strStr(String haystack, String needle) {
    if (haystack == null || needle == null) return -1;

    int haystackLen = haystack.length();
    int needleLen = needle.length();

    if (needleLen > haystackLen) return -1;
    if (needleLen == 0) return 0;

    char[] haystackChars = haystack.toCharArray();
    char[] needleChars = needle.toCharArray();

    int index = 0;
    boolean found = false;
    while (index < haystackLen) {
        if (haystackLen - index < needleLen) {
            break;
        }

        int i = index;

        for (int j = 0; j < needleLen; j++) {
            if (needleChars[j] != haystackChars[i]) {
                break;
            }

            i++;

            if (j == needleLen - 1) {
                found = true;
            }
        }

        if (found) {
            break;
        } else {
            index++;
        }
    }

    return found ? index : -1;
}