added a cpp problem

cpp/1239. Maximum Length of a Concatenated String with Unique Characters.cpp

@@ -0,0 +1,59 @@
+/*
+    Problem Link:- https://leetcode.com/problems/maximum-length-of-a-concatenated-string-with-unique-characters/
+
+    Approach:- backtracking problem
+    we will choose strings with no repetetion of characters if string has repetetion then we will go to next string
+    else
+    if it has no repetetions then we will have two options whether to take it or not for the maximum lengt of strings.
+
+*/
+class Solution 
+{    
+    public:
+    void sol(int &ans,vector<string> &arr,int ind,vector<string> &st,vector<int> &stc)
+    {
+        if(ind>=arr.size())
+            return;
+        vector<int> z(26,0);
+        for(int i=0;i<arr[ind].size();i++)
+            z[arr[ind][i]-'a']++;
+        int f=0;
+        for(int i=0;i<26;i++)
+        {
+            if(stc[i]+z[i]>1)
+            {
+                f=1;
+                break;
+            }
+        }
+        if(f==1)
+        {
+            sol(ans,arr,ind+1,st,stc);
+        }
+        else
+        {
+            int ct=0;
+            for(auto it:st)
+                ct=ct+it.size();
+            ct=ct+arr[ind].size();
+            ans=max(ans,ct);
+            for(int i=0;i<26;i++)
+                stc[i]+=z[i];
+            st.push_back(arr[ind]);
+            sol(ans,arr,ind+1,st,stc);
+            string z1=st.back();
+            for(int i=0;i<z1.size();i++)
+                stc[z1[i]-'a']--;
+            st.pop_back();
+            sol(ans,arr,ind+1,st,stc);
+        }
+    }
+    int maxLength(vector<string>& arr) 
+    {
+        int ans=0;
+        vector<string> st;
+        vector<int> stc(26,0);
+        sol(ans,arr,0,st,stc);
+        return ans;
+    }
+};