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;
    }
};

Discuss

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

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