91. Decode Ways.md

91. Decode Ways

https://leetcode.com/problems/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).

代码

• DP 思路

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