Adding Spaces to a String

Leet code Link - https://leetcode.com/problems/adding-spaces-to-a-string/description/

Complexity - Medium

Description

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

By looking at the solution, we know we have to append the strings o let us use stringbuilder

I can iterate through the spaces array where I can use builtin function substring of the string to get the part of it.

class Solution {
    public String addSpaces(String s, int[] spaces) {
        StringBuilder sb = new StringBuilder();
        int start =0;
        for(int i:spaces){
            sb.append(s.substring(start,i));
            sb.append(" ");
            start=i;
        }
        sb.append(s.substring(start));
        return sb.toString();
    }
}

Complexity

Time Complexity - O(n+m)

Space Complexity - O(1)