Interviewbit/WaystoDecode.cpp at master · jayakrishna-g/Interviewbit · GitHub

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/*
Ways to Decode
Asked in:
Facebook
Amazon
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.


Input Format:

The first and the only argument is a string A.
Output Format:

Return an integer, representing the number of ways to decode the string.
Constraints:

1 <= length(A) <= 1e5
Example :

Input 1:
    A = "8"

Output 1:
    1

Explanation 1:
    Given encoded message "8", it could be decoded as only "H" (8).

    The number of ways decoding "8" is 1.

Input 2:
    A = "12"

Output 2:
    2

Explanation 2:
    Given encoded message "12", it could be decoded as "AB" (1, 2) or "L" (12).

    The number of ways decoding "12" is 2.
Seen this question in a real interview before
*/

long  dp[100010];
long foo(int ai,string &A)
{
    if(ai>A.size())
    return 0;
    if(ai==A.size())
    return 1;
    if(dp[ai]!=-1)
    return dp[ai];
    if(A[ai]=='0')
    {
        return 0;
    }
    if(A[ai]=='1')
    {
        if(ai!=A.size()-1 && A[ai+1]==0)
        return dp[ai]=foo(ai+2,A);
        return dp[ai]=foo(ai+1,A)+foo(ai+2,A);
    }
    if(A[ai]=='2')
    {

        if(ai!=A.size()-1 && A[ai+1]==0)
        return dp[ai]=foo(ai+2,A);
        if(ai!=A.size()-1 && A[ai+1]<='6')
        return dp[ai]=foo(ai+1,A)+foo(ai+2,A);
        return dp[ai]=foo(ai+1,A);
    }
    return dp[ai]=foo(ai+1,A);
}
int Solution::numDecodings(string A) {
    memset(dp,-1,sizeof(dp));
    return foo(0,A);
}