1309. Decrypt String from Alphabet to Integer Mapping

Problem Description

The problem asks to decrypt a string s where digits map to lowercase letters. Digits '1' to '9' map to 'a' to 'i', respectively. Two-digit numbers followed by '#' map to 'j' to 'z'. The function should return the decrypted string.

Solution: Simulation

The most straightforward approach is to simulate the decryption process. We iterate through the string:

1. Check for two-digit numbers: If we encounter a '#' character and have at least two digits before it, we interpret those two digits as a number between 10 and 26. This number is then used to determine the corresponding character ('j' to 'z').
2. Handle single-digit numbers: If we don't find a '#' or only have one digit, we treat it as a single-digit number (1-9) and map it to the appropriate character ('a' to 'i').

We build the decrypted string character by character and return it.

Code Implementation (Python)

class Solution:
    def freqAlphabets(self, s: str) -> str:
        res = ""
        i = 0
        n = len(s)
        while i < n:
            if i + 2 < n and s[i+2] == '#':
                num = int(s[i:i+2])
                res += chr(ord('a') + num - 1)
                i += 3
            else:
                num = int(s[i])
                res += chr(ord('a') + num - 1)
                i += 1
        return res

Time and Space Complexity Analysis

• Time Complexity: O(n), where n is the length of the input string s. We iterate through the string once.
• Space Complexity: O(n) in the worst case. The output string res can be at most the same length as the input string.

Code Implementation in Other Languages

The solution can be easily adapted to other programming languages using similar logic. The core idea remains the same: iterate through the string, check for the '#' character to handle two-digit mappings, and build the resulting string.

(Java)

class Solution {
    public String freqAlphabets(String s) {
        StringBuilder sb = new StringBuilder();
        int i = 0;
        while (i < s.length()) {
            if (i + 2 < s.length() && s.charAt(i + 2) == '#') {
                int num = Integer.parseInt(s.substring(i, i + 2));
                sb.append((char) (num + 96)); // 'a' is 97 in ASCII
                i += 3;
            } else {
                int num = Integer.parseInt(s.substring(i, i + 1));
                sb.append((char) (num + 96));
                i++;
            }
        }
        return sb.toString();
    }
}

(C++)

string freqAlphabets(string s) {
    string res = "";
    for (int i = 0; i < s.length(); ++i) {
        if (i + 2 < s.length() && s[i + 2] == '#') {
            int num = stoi(s.substr(i, 2));
            res += (char)('a' + num - 1);
            i += 2;
        } else {
            int num = s[i] - '0';
            res += (char)('a' + num - 1);
        }
    }
    return res;
}

The other languages (JavaScript, Go, C#, etc.) would follow a similar pattern, adapting the string manipulation and character conversion functions appropriately.