Implement strStr()

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 whenneedleis an empty string? This is a great question to ask during an interview.

For the purpose of this problem, we will return 0 whenneedleis an empty string. This is consistent to C's strstr() and Java's indexOf().

Solution

class Solution {
    // "aaaaa", "b" => -1
    // "aaaaa", "a" => 0
    // "aa", "aa" => 0
    // hello, ll => 2
    public int strStr(String haystack, String needle) {
        int index = -1;
        if (needle == null || haystack == null) {
            return index;
        }
        if (needle.length() > haystack.length()) {
            return index;
        }
        if (needle.equals(haystack)) {
            return 0;
        }

        for (int i = 0; i < haystack.length(); i++) {
            if (i + needle.length() <= haystack.length()) {
                // for example: hello & ll
                // 2 + 2 <= 5, try to compare substring(2, 2 + 2) == needle
                if (haystack.substring(i, i + needle.length()).equals(needle)) {
                    return i;
                }    
            }
        }
        return index;
    }
}