28. Implement strStr()

Implement strStr().

Returns the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.

Solution:

Method 1: Brute Force

An optimized brute force implementation.

The time complexity is O(n²).



Code:

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

Method 2 : Booye-Moore

The idea is to pre-process a table that when we do the compare of two strings, we know how many steps to skip and continue comparing.

The best time complexity is O(m / n) and worst cast time complexity is O(mn).

The space complexity is O(1).



Code:

public class Solution {
    public int strStr(String haystack, String needle) {
        int m = haystack.length();
        int n = needle.length();
        int[] right = new int[256];
        Arrays.fill(right, -1);
        for (int j = 0; j < n; j++) {
            right[needle.charAt(j)] = j;
        }
        int skip = 0;
        for (int i = 0; i < m - n + 1; i++) {
            skip = 0;
            for (int j = n - 1; j >= 0; j--) {
                if (haystack.charAt(i + j) != needle.charAt(j)) {
                    skip = Math.max(1, right[haystack.charAt(i + j)]);
                    break;
                }
            }
            if (skip == 0) {
                return i;
            }
        }
        return -1;
    }
}