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.

class Solution {
public:
    string addSpaces(string s, vector<int>& spaces) {
        string res = "";
        for(int i = s.length() - 1; i >= 0; i--) {
            res.push_back(s[i]);
            if(!spaces.empty() and spaces.back() == i) {
                res.push_back(' ');
                spaces.pop_back();
            }
        }
        reverse(begin(res), end(res));
        return res;
    }
};