2109. Adding Spaces to a String

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.

C++

// Time: O(N + M) = O(N)
// Space: O(1) extra space
class Solution {
public:
    string addSpaces(string s, vector<int>& A) {
        string ans;
        ans.reserve(s.size() + A.size()); 
        int N = s.size(), j = 0, M = A.size(); 
        for (int i = 0; i < N; ++i) {
            if (j < M && i == A[j]) {
                ans += ' ';
                ++j;
            }
            ans += s[i];
        }
        return ans;
    }
};

JAVA

• class Solution {
      public String addSpaces(String s, int[] spaces) {
          StringBuilder ans = new StringBuilder();
          for (int i = 0, j = 0; i < s.length(); ++i) {
              if (j < spaces.length && i == spaces[j]) {
                  ans.append(' ');
                  ++j;
              }
              ans.append(s.charAt(i));
          }
          return ans.toString();
      }
  }

PYTHON

• class Solution:
      def addSpaces(self, s: str, spaces: list[int]) -> str:
          ans = []
          j, m = 0, len(spaces) 
          for i, ch in enumerate(s):
              if j < m and i == spaces[j]:
                  ans.append(' ')
                  j += 1
              ans.append(ch)
          return ''.join(ans)