Decode Ways
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A message containing letters from A-Z
is being encoded to numbers using the following mapping:
'A' -> 1'B' -> 2...'Z' -> 26
Given an encoded message containing digits, determine the total number of ways to decode it.
For example,
Given encoded message "12"
, it could be decoded as "AB"
(1 2) or"L"
(12).
The number of ways decoding "12"
is 2.
class Solution {public: int numDecodings(string s) { if(!s.size())return 0;if(s[0] == '0')return 0;int *dp = new int[s.size() + 1];dp[0] = 1;for(int i = 1; i <= s.size(); i++){if(i >= 2 && s[i - 1] == '0'){//注意..00 ..50的情况 返回0if(s[i - 2] > '2' || s[i - 2] == '0')return 0;dp[i] = dp[i - 2];//..20 (..10) 0只能和1(2)组合continue;}dp[i] = dp[i - 1]; //..AX X单独自己一个单词//..2X x<6 //..1X 1和X组合if(i >= 2 && (s[i - 2] == '1' || (s[i - 2] == '2' && s[i - 1] <= '6'))){ dp[i] += dp[i - 2]; }} return dp[s.size()]; }};
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