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).

91. Decode Ways
``````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 = 1;
a = if s > 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);
}
``````