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Java-based LeetCode algorithm problem solutions, regularly updated

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package g0001_0100.s0091_decode_ways;

// #Medium #Top_Interview_Questions #String #Dynamic_Programming
// #Algorithm_II_Day_15_Dynamic_Programming #Dynamic_Programming_I_Day_10
// #2022_06_21_Time_2_ms_(66.37%)_Space_41.8_MB_(78.45%)

/**
 * 91 - Decode Ways\.
 *
 * Medium
 *
 * A message containing letters from `A-Z` can be **encoded** into numbers using the following mapping:
 *
 * 'A' -> "1" 'B' -> "2" ... 'Z' -> "26" 
 *
 * To **decode** an encoded message, all the digits must be grouped then mapped back into letters using the reverse of the mapping above (there may be multiple ways). For example, `"11106"` can be mapped into:
 *
 * *   `"AAJF"` with the grouping `(1 1 10 6)`
 * *   `"KJF"` with the grouping `(11 10 6)`
 *
 * Note that the grouping `(1 11 06)` is invalid because `"06"` cannot be mapped into `'F'` since `"6"` is different from `"06"`.
 *
 * Given a string `s` containing only digits, return _the **number** of ways to **decode** it_.
 *
 * The answer is guaranteed to fit in a **32-bit** integer.
 *
 * **Example 1:**
 *
 * **Input:** s = "12"
 *
 * **Output:** 2
 *
 * **Explanation:** "12" could be decoded as "AB" (1 2) or "L" (12). 
 *
 * **Example 2:**
 *
 * **Input:** s = "226"
 *
 * **Output:** 3
 *
 * **Explanation:** "226" could be decoded as "BZ" (2 26), "VF" (22 6), or "BBF" (2 2 6). 
 *
 * **Example 3:**
 *
 * **Input:** s = "0"
 *
 * **Output:** 0
 *
 * **Explanation:** There is no character that is mapped to a number starting with 0. The only valid mappings with 0 are 'J' -> "10" and 'T' -> "20", neither of which start with 0. Hence, there are no valid ways to decode this since all digits need to be mapped. 
 *
 * **Example 4:**
 *
 * **Input:** s = "06"
 *
 * **Output:** 0
 *
 * **Explanation:** "06" cannot be mapped to "F" because of the leading zero ("6" is different from "06"). 
 *
 * **Constraints:**
 *
 * *   `1 <= s.length <= 100`
 * *   `s` contains only digits and may contain leading zero(s).
**/
public class Solution {
    public int numDecodings(String s) {
        if (s.charAt(0) == '0') {
            return 0;
        }
        int n = s.length();
        int[] f = new int[n + 1];
        // Auxiliary
        f[0] = 1;
        f[1] = 1;
        for (int i = 2; i <= n; i++) {
            // Calculate the independent number
            if (s.charAt(i - 1) != '0') {
                // As long as the current character is not 0, it means that the previous decoding
                // number can be inherited
                f[i] = f[i - 1];
            }
            // Calculate the number of combinations
            int twodigits = (s.charAt(i - 2) - '0') * 10 + (s.charAt(i - 1) - '0');
            if (twodigits >= 10 && twodigits <= 26) {
                f[i] += f[i - 2];
            }
        }
        return f[n];
    }
}