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`

.our__Y__offee"**C**

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 "LeetcodeelpsHeMearn". We then place spaces before those characters.L

**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 "iodecniyphon". We then place spaces before those characters.t

**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 * 10`

^{5}`s`

consists only of lowercase and uppercase English letters.`1 <= spaces.length <= 3 * 10`

^{5}`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;
}
};
```

## Discuss

https://leetcode.com/problems/adding-spaces-to-a-string/discuss/1635042/C%2B%2B-Straightforward