Decode_ways.cpp

/*

Link : https://leetcode.com/problems/decode-ways/

Problem Statement :
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
*/

//Solution :

class Solution {
public:
    int numDecodings(string s) {
        if(s.size() == 0) return 0;
        vector<int> dp(s.size() + 1);
        dp[s.size()] = 1;
        for(int i = s.size() - 1; i >= 0; i--) {
            if(s[i] == '0')
                dp[i] = 0;
            else {
                dp[i] += dp[i+1];
                if(i < s.size() - 1 && (s[i] == '1' || s[i] == '2' && s[i+1] < '7')) dp[i] += dp[i+2];
            }
        }
        return dp[0];
    }
};