28. Implement strStr().md

Solution1 (brute force)

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

note

• time O(m * n) space O(1)
• The trick here in to adding j to the haystack index while moving forward to compare since our i is constant when the inner loop is on the fly. Also, we need to check if i + j == haystack.length() during every inner iteration to prevent IndexOutOfBounds.