796. Rotate String

Difficulty: Easy

Topics: String, String Matching

Given two strings s and goal, return true if and only if s can become goal after some number of shifts on s.

A shift on s consists of moving the leftmost character of s to the rightmost position.

For example, if s = "abcde", then it will be "bcdea" after one shift.

Example 1:

Input: s = "abcde", goal = "cdeab"
Output: true

Example 2:

Input: s = "abcde", goal = "abced"
Output: false

Constraints:

1 <= s.length, goal.length <= 100
s and goal consist of lowercase English letters.

Solution:

We can take advantage of the properties of string concatenation. Specifically, if we concatenate the string s with itself (i.e., s + s), all possible rotations of s will appear as substrings within that concatenated string. This allows us to simply check if goal is a substring of s + s.

Let's implement this solution in PHP: 796. Rotate String

<?php
/**
 * @param String $s
 * @param String $goal
 * @return Boolean
 */
function rotateString($s, $goal) {
    ...
    ...
    ...
    /**
     * go to ./solution.php
     */
?>

Explanation:

Length Check: We first check if the lengths of s and goal are the same. If they are not, we immediately return false, as it's impossible for s to be transformed into goal.
Concatenation: We concatenate the string s with itself to create doubleS.
Substring Check: We use the strpos() function to check if goal exists as a substring in doubleS. If it does, we return true; otherwise, we return false.

Complexity:

Time Complexity: O(n), where n is the length of the string, due to the concatenation and substring search.
Space Complexity: O(n) for the concatenated string.

This solution efficiently determines if one string can become another through rotations.

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