2109. Adding Spaces to a String

2109. Adding Spaces to a String

Difficulty: Medium

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).

Solution:

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: a star on GitHub or sharing the post on your favorite social networks 😍.