Shifting Letters

LeetCode Q 848 - Shifting Letters

We have a string S of lowercase letters, and an integer array shifts.
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 1: 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.

Note:

• 1 <= S.length = shifts.length <= 20000
• 0 <= shifts[i] <= 10 ^ 9

Solution

Key Point:
When accumulating the shift numbers, to avoid Integer Overflow, we store mod value in shifts.

Code:

public String shiftingLetters(String S, int[] shifts) {
  int len = shifts.length;
  for (int i = len - 2; i >= 0; i--) {
    shifts[i] += shifts[i + 1];
    shifts[i] %= 26;
  }
    
  
  char[] chs = S.toCharArray();
  for (int i = 0; i < len; i++) {
    chs[i] = (char)((chs[i] - 'a' + shifts[i]) % 26 + 'a');
  }

  return new String(chs);
}