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Daily/Number_of_distrinct_sequence.cpp

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+/*
+Given a string consisting of lower case English alphabets, the task is to find the number 
+of distinct subsequences of the string
+Note: Answer can be very large, so, ouput will be answer modulo 109+7.
+
+Example 1:
+
+Input: 
+s = "gfg"
+Output: 
+7
+Explanation: 
+The seven distinct subsequences are "", "g", "f", "gf", "fg", "gg" and "gfg" .
+
+Example 2:
+
+Input: 
+s = "ggg"
+Output: 
+4
+Explanation: 
+The four distinct subsequences are "", "g", "gg", "ggg". */
+
+//{ Driver Code Starts
+#include <bits/stdc++.h>
+using namespace std;
+
+
+// } Driver Code Ends
+//User function template for C++
+
+class Solution{
+  public:	
+	#define ll long long
+    const ll mod=1e9+7;
+	int distinctSubsequences(string s) {
+	    int n=s.size();
+	    vector<ll> pos(26,n);
+	    vector<ll> dp(n+1,0);
+	    ll ans=1;
+	    for(int i=n-1;i>=0;i--){
+	        ll cur = dp[i+1];
+	        if(pos[s[i]-'a'] < n)
+	            cur=(cur-dp[pos[s[i]-'a']+1]+mod)%mod;
+	        else
+	            cur=(cur+1)%mod;
+	        dp[i]=(cur+dp[i+1])%mod;
+	        pos[s[i]-'a']=i;
+	    }
+	    return dp[0]+1;
+	}
+};
+
+//{ Driver Code Starts.
+
+int main() 
+{
+
+
+   	ios_base::sync_with_stdio(0);
+    cin.tie(NULL);
+    cout.tie(NULL);
+
+   	int t;
+   	cin >> t;
+   	while(t--)
+   	{
+   		string s;
+   		cin >> s;
+
+        Solution ob;
+   		cout << ob.distinctSubsequences(s) << "\n";
+   	}
+
+    return 0;
+}
+// } Driver Code Ends