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).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);
}