g0001_0100.s0028_implement_strstr.Solution Maven / Gradle / Ivy

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package g0001_0100.s0028_implement_strstr;

// #Easy #Top_Interview_Questions #String #Two_Pointers #String_Matching
// #Programming_Skills_II_Day_1 #2022_06_14_Time_0_ms_(100.00%)_Space_40.4_MB_(83.88%)

/**
 * 28 - Implement strStr()\.
 *
 * Easy
 *
 * Implement [strStr()](http://www.cplusplus.com/reference/cstring/strstr/).
 *
 * Return the index of the first occurrence of needle in haystack, or `-1` if `needle` is not part of `haystack`.
 *
 * **Clarification:**
 *
 * What should we return when `needle` is an empty string? This is a great question to ask during an interview.
 *
 * For the purpose of this problem, we will return 0 when `needle` is an empty string. This is consistent to C's [strstr()](http://www.cplusplus.com/reference/cstring/strstr/) and Java's [indexOf()](https://docs.oracle.com/javase/7/docs/api/java/lang/String.html#indexOf(java.lang.String)).
 *
 * **Example 1:**
 *
 * **Input:** haystack = "hello", needle = "ll"
 *
 * **Output:** 2 
 *
 * **Example 2:**
 *
 * **Input:** haystack = "aaaaa", needle = "bba"
 *
 * **Output:** -1 
 *
 * **Example 3:**
 *
 * **Input:** haystack = "", needle = ""
 *
 * **Output:** 0 
 *
 * **Constraints:**
 *
 * *   0 <= haystack.length, needle.length <= 5 * 10⁴
 * *   `haystack` and `needle` consist of only lower-case English characters.
**/
public class Solution {
    public int strStr(String haystack, String needle) {
        if (needle.isEmpty()) {
            return 0;
        }
        int m = haystack.length();
        int n = needle.length();
        for (int start = 0; start < m - n + 1; start++) {
            if (haystack.substring(start, start + n).equals(needle)) {
                return start;
            }
        }
        return -1;
    }
}