2000. Reverse Prefix of Word

Easy

Given a 0-indexed string word and a character ch, reverse the segment of word that starts at index 0 and ends at the index of the first occurrence of ch (inclusive). If the character ch does not exist in word, do nothing.

For example, if word = "abcdefd" and ch = "d", then you should reverse the segment that starts at 0 and ends at 3 (inclusive). The resulting string will be "dcbaefd".

Return t*he resulting string*.

Example 1:

Input: word = "abcdefd", ch = "d"
Output: "dcbaefd"
Explanation: The first occurrence of "d" is at index 3.\ Reverse the part of word from 0 to 3 (inclusive), the resulting string is "dcbaefd".

Example 2:

Input: word = "xyxzxe", ch = "z"
Output: "zxyxxe"
Explanation: The first and only occurrence of "z" is at index 3.\ Reverse the part of word from 0 to 3 (inclusive), the resulting string is "zxyxxe".

Example 3:

Input: word = "abcd", ch = "z"
Output: "abcd"
Explanation: "z" does not exist in word.\ You should not do any reverse operation, the resulting string is "abcd".

Constraints:

1 <= word.length <= 250
word consists of lowercase English letters.
ch is a lowercase English letter.

Solution:

class Solution {

    /**
     * @param String $word
     * @param String $ch
     * @return String
     */
    function reversePrefix($word, $ch) {
        $pos = strpos($word, $ch);
        if ($pos !== false) {
            $prefix = substr($word, 0, $pos + 1);
            $reversedPrefix = strrev($prefix);
            $rest = substr($word, $pos + 1);
            return $reversedPrefix . $rest;
        }
        return $word;
    }
}

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