Formatted question description: https://leetcode.ca/all/2109.html

# 2109. Adding Spaces to a String (Medium)

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.

## Solution 1. Two Pointers

// OJ: https://leetcode.com/problems/adding-spaces-to-a-string/
// 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()); // Optional: pre-allocate enough space for the answer.
int N = s.size(), j = 0, M = A.size(); // A[j] is the next index before which a space should be inserted
for (int i = 0; i < N; ++i) {
if (j < M && i == A[j]) { // If i == A[j], we insert a space
ans += ' ';
++j;
}
ans += s[i];
}
return ans;
}
};