28. Implement strStr()

/**
 * Brute Force string matching.
 *
 * Time Complexity: O(M * N)
 *
 * Space Complexity: O(1)
 *
 * M = Length of haystack string. N = length of fneedle string.
 */
class Solution1 {
    public int strStr(String haystack, String needle) {
        if (needle.isEmpty()) {
            return 0;
        }
        for (int i = 0; i <= haystack.length() - needle.length(); i++) {
            for (int j = 0; j < needle.length(); j++) {
                if (haystack.charAt(i + j) != needle.charAt(j)) {
                    break;
                }
                if (j == needle.length() - 1) {
                    return i;
                }
            }
        }
        return -1;
    }
}

/**
 * Using KMP Algorithm
 *
 * Time Complexity: O(M + N). O(N) to create lookup table. O(M) to find the
 * needle in haystack.
 *
 * Space Complexity: O(N). This is required to save the lookup table.
 *
 * M = Length of haystack string. N = length of fneedle string.
 */
class Solution2 {
    public int strStr(String haystack, String needle) {
        if (haystack == null || needle == null) {
            return -1;
        }
        if (needle.length() == 0) {
            return 0;
        }
        if (haystack.length() == 0) {
            return -1;
        }

        int[] table = kmpLookupTable(needle);

        int i = 0;
        int j = 0;
        while (i < haystack.length() && j < needle.length()) {
            if (haystack.charAt(i) == needle.charAt(j)) {
                i++;
                j++;
            } else {
                if (j > 0) {
                    j = table[j - 1];
                } else {
                    i++;
                }
            }
        }
        if (j == needle.length()) {
            return i - j;
        } else {
            return -1;
        }
    }

    private int[] kmpLookupTable(String s) {
        int[] table = new int[s.length()];
        int i = 1;
        int index = 0;
        while (i < s.length()) {
            if (s.charAt(i) == s.charAt(index)) {
                table[i] = index + 1;
                index++;
                i++;
            } else {
                if (index > 0) {
                    index = table[index - 1];
                } else {
                    table[i] = 0;
                    i++;
                }
            }
        }
        return table;
    }
}