76. Decode Ways

class Solution:
    def numDecodings(self, s: str) -> int:
        prev2, prev1 = 1, 1 if s[0] != '0' else 0      # ways for s[:i-2], s[:i-1]
        for i in range(1, len(s)):
            cur = 0
            if s[i] != '0':
                cur += prev1                           # take one digit
            if 10 <= int(s[i - 1:i + 1]) <= 26:
                cur += prev2                           # take two digits
            prev2, prev1 = prev1, cur
        return prev1

class Solution {
    public int numDecodings(String s) {
        int prev2 = 1, prev1 = s.charAt(0) != '0' ? 1 : 0;
        for (int i = 1; i < s.length(); i++) {
            int cur = 0, two = Integer.parseInt(s.substring(i - 1, i + 1));
            if (s.charAt(i) != '0') cur += prev1;     // one digit
            if (two >= 10 && two <= 26) cur += prev2; // two digits
            prev2 = prev1; prev1 = cur;
        }
        return prev1;
    }
}

class Solution {
public:
    int numDecodings(string s) {
        int prev2 = 1, prev1 = s[0] != '0' ? 1 : 0;
        for (size_t i = 1; i < s.size(); i++) {
            int cur = 0, two = (s[i - 1] - '0') * 10 + (s[i] - '0');
            if (s[i] != '0') cur += prev1;            // one digit
            if (two >= 10 && two <= 26) cur += prev2; // two digits
            prev2 = prev1; prev1 = cur;
        }
        return prev1;
    }
};

function numDecodings(s) {
  let prev2 = 1, prev1 = s[0] !== '0' ? 1 : 0;
  for (let i = 1; i < s.length; i++) {
    let cur = 0;
    const two = Number(s.slice(i - 1, i + 1));
    if (s[i] !== '0') cur += prev1;            // one digit
    if (two >= 10 && two <= 26) cur += prev2;  // two digits
    [prev2, prev1] = [prev1, cur];
  }
  return prev1;
}