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2109 - Adding Spaces to a String (Medium)

https://leetcode.com/problems/adding-spaces-to-a-string/

Problem Statement​

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 * 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.

Approach 1: Two Pointers​

We iterate the string using pointer i and iterate spaces using pointer j. If i is equal to spaces[j] , then we need to add a space there and increase pointer j.

Written by @wingkwong
class Solution {
public:
    string addSpaces(string s, vector<int>& spaces) {
        string ans;
        int j = 0, m = spaces.size();
        for (int i = 0; i < s.size(); i++) {
            if (j < m && i == spaces[j]) ans += " ", j++;
            ans += s[i];
        }
        return ans;
    }
};