[LeetCode] Shifting Letters

Problem: You are given a string s of lowercase English letters and an integer array shifts of the same length.

Call the shift() of a letter, the next letter in the alphabet, (wrapping around so that 'z' becomes 'a').

For example, shift('a') = 'b', shift('t') = 'u', and shift('z') = 'a'.

Now for each shifts[i] = x, we want to shift the first i + 1 letters of s, x times.

Return the final string after all such shifts to s are applied.

Example:

Input: s = "abc", shifts = [3,5,9]
Output: "rpl"
Explanation: We start with "abc".
After shifting the first 1 letters of s by 3, we have "dbc".
After shifting the first 2 letters of s by 5, we have "igc".
After shifting the first 3 letters of s by 9, we have "rpl", the answer.

Input: s = "aaa", shifts = [1,2,3]
Output: "gfd"

Constraints:

• 1 <= s.length <= 105
• s consists of lowercase English letters.
• shifts.length == s.length
• 0 <= shifts[i] <= 109

Approach: The approach is very much straight forward. The only problem is while shifting characters we need to take care of a case where s[i] + shifts[i] is more than the value of 'z'. We can take care of this problem by using mod 26.

Implementation in C#:

    public string ShiftingLetters(string s, int[] shifts) 

    {

        int length = shifts.Length;

        StringBuilder stringBuilder = new StringBuilder();

        shifts[length - 1] %= 26;

        for (int i = length - 2; i >= 0; --i)

        {

            shifts[i] = (shifts[i + 1] + shifts[i]) % 26;

        }

        for (int i = 0; i < length; ++i)

        {

            int pos = s[i] - 'a';

            stringBuilder.Append((char)(((pos + shifts[i]) % 26) + 'a'));

        }    

        return stringBuilder.ToString();

    }

Complexity: O(n)