91. Decode Ways

A message containing letters from A-Z is being encoded to numbers using the following mapping:

'A' -> 1
'B' -> 2
...
'Z' -> 26

Given a non-empty string containing only digits, determine the total 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: It could be decoded as "AB" (1 2) or "L" (12).

Example 2:

Input: s = "226"
Output: 3
Explanation: It 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'. We cannot ignore a zero when we face it while decoding. So, each '0' should be part of "10" --> 'J' or "20" --> 'T'.

Example 4:

Input: s = "1"
Output: 1

 

Constraints:

  • 1 <= s.length <= 100
  • s contains only digits and may contain leading zero(s).

Rust Solution

struct Solution;

impl Solution {
    fn num_decodings(s: String) -> i32 {
        let s: Vec<u8> = s.bytes().map(|b| b - b'0').collect();
        let n = s.len();
        let mut a: Vec<i32> = vec![0; n + 1];
        if n == 0 {
            return 0;
        }
        a[0] = 1;
        a[1] = if s[0] > 0 { 1 } else { 0 };
        for i in 1..n {
            let first = s[i];
            let second = s[i - 1] * 10 + s[i];
            if (1..=9).contains(&first) {
                a[i + 1] += a[i];
            }
            if (10..=26).contains(&second) {
                a[i + 1] += a[i - 1];
            }
        }
        a[n]
    }
}

#[test]
fn test() {
    let s = "12".to_string();
    assert_eq!(Solution::num_decodings(s), 2);
    let s = "226".to_string();
    assert_eq!(Solution::num_decodings(s), 3);
}

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