#91

Decode Ways

medium· 1-D DPruns: 0

A message containing letters from A-Z can be encoded into numbers using the mapping 'A' to 1, 'B' to 2, ..., 'Z' to 26. To decode an encoded message, the digits must be grouped and mapped back into letters. Given a string s of digits, return the number of ways to decode it. The test cases are generated so the answer fits in a 32-bit integer.

wpm 0acc 100%time 0:000 / 405

class Solution:
    def numDecodings(self, s: str) -> int:
        if not s or s[0] == "0":
            return 0
        prev, curr = 1, 1
        for i in range(1, len(s)):
            new = 0
            if s[i] != "0":
                new += curr
            two = int(s[i - 1 : i + 1])
            if 10 <= two <= 26:
                new += prev
            prev, curr = curr, new
        return curr

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