2000. Reverse Prefix of Word

Problem

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 the 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

/**
 * @param {string} word
 * @param {character} ch
 * @return {string}
 */
var reversePrefix = function(word, ch) {
    var index = word.indexOf(ch);
    if (index === -1) return word;
    var arr = word.split('');
    for (var i = 0; i < index / 2; i++) {
        var tmp = arr[i];
        arr[i] = arr[index - i];
        arr[index - i] = tmp;
    }
    return arr.join('');
};

Explain:

nope.

Complexity:

• Time complexity : O(n).
• Space complexity : O(n).