dp[i] represents the number of ways to decode s[:i]. At each position, we check if a single digit or a two-digit number forms a valid decoding.

1. If s[0] == '0', return 0.
2. dp[0] = 1, dp[1] = 1.
3. For each position i from 2 to n:
   • If s[i-1] != '0', add dp[i-1] (single digit decode).
   • If 10 <= int(s[i-2:i]) <= 26, add dp[i-2] (two digit decode).
4. Return dp[n].

• Time Complexity: O(N).
• Space Complexity: O(N), reducible to O(1).

def num_decodings(s):
    if not s or s[0] == '0':
        return 0
    
    n = len(s)
    dp = [0] * (n + 1)
    dp[0] = 1
    dp[1] = 1
    
    for i in range(2, n + 1):
        if s[i - 1] != '0':
            dp[i] += dp[i - 1]
        two_digit = int(s[i - 2:i])
        if 10 <= two_digit <= 26:
            dp[i] += dp[i - 2]
    
    return dp[n]