2109. Adding Spaces to a String

[mah-shamim]

Topics: Array, Two Pointers, String, Simulation

You are given a 0-indexed string s and a 0-indexed integer array spaces that describes the indices in the original string where spaces will be added. Each space should be inserted before the character at the given index.

• For example, given s = "EnjoyYourCoffee" and spaces = [5, 9], we place spaces before 'Y' and 'C', which are at indices 5 and 9 respectively. Thus, we obtain "Enjoy Your Coffee".

Return the modified string after the spaces have been added.

Example 1:

• Input: s = "LeetcodeHelpsMeLearn", spaces = [8,13,15]
• Output: "Leetcode Helps Me Learn"
• Explanation: The indices 8, 13, and 15 correspond to the underlined characters in "LeetcodeHelpsMeLearn".
  We then place spaces before those characters.

Example 2:

• Input: s = "icodeinpython", spaces = [1,5,7,9]
• Output: "i code in py thon"
• Explanation: The indices 1, 5, 7, and 9 correspond to the underlined characters in "icodeinpython".
  We then place spaces before those characters.

Example 3:

• Input: s = "spacing", spaces = [0,1,2,3,4,5,6]
• Output: " s p a c i n g"
• Explanation: We are also able to place spaces before the first character of the string.

Constraints:

• 1 <= s.length <= 3 * 105
• s consists only of lowercase and uppercase English letters.
• 1 <= spaces.length <= 3 * 105
• 0 <= spaces[i] <= s.length - 1
• All the values of spaces are strictly increasing.

Hint:

1. Create a new string, initially empty, as the modified string. Iterate through the original string and append each character of the original string to the new string. However, each time you reach a character that requires a space before it, append a space before appending the character.
2. Since the array of indices for the space locations is sorted, use a pointer to keep track of the next index to place a space. Only increment the pointer once a space has been appended.
3. Ensure that your append operation can be done in O(1).

[mah-shamim]

We can use an efficient approach with two pointers. Here's how the implementation in PHP 5.6 would look:

Solution Explanation:

1. Use a pointer spaceIndex to track the current position in the spaces array.
2. Iterate through the string s using a loop.
3. Check whether the current index in the string matches the current value in the spaces array. If it does, append a space to the result and move the spaceIndex pointer forward.
4. Append the current character of the string to the result.
5. Return the final result as a single string.

This approach ensures that we process the input efficiently, taking advantage of the sorted order of the spaces array.

Let's implement this solution in PHP: 2109. Adding Spaces to a String

<?php
 /**
 * @param String $s
 * @param Integer[] $spaces
 * @return String
 */
function addSpaces($s, $spaces) {
    $result = ""; // Initialize the resulting string
    $spaceIndex = 0; // Pointer for the spaces array
    $n = strlen($s); // Length of the string
    $spacesCount = count($spaces); // Number of spaces to add

    for ($i = 0; $i < $n; $i++) {
        // If the current index matches the space index, add a space
        if ($spaceIndex < $spacesCount && $i == $spaces[$spaceIndex]) {
            $result .= " ";
            $spaceIndex++;
        }
        // Append the current character to the result
        $result .= $s[$i];
    }

    return $result;
}

// Example 1
$s1 = "LeetcodeHelpsMeLearn";
$spaces1 = [8, 13, 15];
echo addSpaces($s1, $spaces1) . "\n"; // Output: "Leetcode Helps Me Learn"

// Example 2
$s2 = "icodeinpython";
$spaces2 = [1, 5, 7, 9];
echo addSpaces($s2, $spaces2) . "\n"; // Output: "i code in py thon"

// Example 3
$s3 = "spacing";
$spaces3 = [0, 1, 2, 3, 4, 5, 6];
echo addSpaces($s3, $spaces3) . "\n"; // Output: " s p a c i n g"
?>

Explanation:

1. Efficient Appending: The . operator in PHP is used to append strings efficiently.
2. Two Pointers: The spaceIndex pointer ensures we only process the spaces array once.
3. Time Complexity:
   • Iterating over the string takes O(n), where n is the length of the string.
   • Checking against the spaces array pointer takes O(m), where m is the length of the spaces array.
   • Combined: O(n + m), which is optimal given the constraints.

This solution adheres to the constraints and is efficient even for large inputs.

[kovatz]

Thanks for solving the problem of adding spaces to a string

[mah-shamim]

Thanks