Q32 Number of distinct subsequences | Practice | GeeksforGeeks

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Sun Mar 26 2023 10:37:44 GMT+0000 (Coordinated Universal Time)

Saved by @Ayush_dabas07

//{ Driver Code Starts
//Initial Template for Java

import java.io.*;
import java.util.*;

class GFG {
    public static void main(String args[]) throws IOException {
        BufferedReader read =
            new BufferedReader(new InputStreamReader(System.in));
        int t = Integer.parseInt(read.readLine());
        while (t-- > 0) {
            String s = read.readLine();
            Solution ob = new Solution();
            System.out.println(ob.distinctSubsequences(s));
        }
    }
}
// } Driver Code Ends


//User function Template for Java

class Solution {
    int distinctSubsequences(String s) {
        //not getting submitted on GFG cause not included mod operations
        //every cell denotes no. of distinct subsequences(NODS) till that string
        
        int n = s.length();
        
        //n+1 , as 0th place would be taken by empty subsequence
        int dp[] = new int[n+1];    
        dp[0] = 1 ;//for 1 character string , NODS = 1
        
        //make HM to store latest indexes of repeated characters
        HashMap<Character ,Integer> map = new HashMap<>();
        
        for(int i = 1 ; i < n+1 ; i++){
            
            dp[i] = 2*dp[i-1];
            
            char ch = s.charAt(i-1);  //dp is 1 greater than string
            
            /* if character already exists means its repeated, so we will subtract all
            subsequences of dp[ch - 1] where ch represent last occurence of repeated 
            character before now, as those are the ones which will get combined and repeated
            */
            if(map.containsKey(ch)) {
                int idx = map.get(ch);  //getting previous occurence of repeated character
                dp[i] -= dp[idx- 1];
            }
            
            map.put(ch , i);    //updating characters
        }
        
        return dp[n];
    }
}

https://practice.geeksforgeeks.org/problems/number-of-distinct-subsequences0909/1

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@Ayush_dabas07

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