Set sub sequences., C++ (clang)

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//clang 3.8.0

#include <iostream>
#include <vector>
const int MAX_CHAR = 256;

int countSub(std::string str);
int main()
{
    
   // std::string str="Hello";
    //const char  *item = str.c_str();
    std::string str;
    std::cin>>str;     
    int testCase;
    std::cin>>testCase;
    
    
    while(testCase--){
        int start=0;
        int end=0;
        std::cin>>start>>end;
        std::string concat2;
        for(int i=start-1;i<end-1;i++){
            concat2+=str[i];
           // std::cout<<concat2;
         }  
        int output = countSub(concat2);
        std::cout<<output<<std::endl;
            
    }
}

int countSub(std::string str){
    
    
    std::vector<int> last(MAX_CHAR,-1);
    
    int n = str.length();
    int dp[n+1];
    
    dp[0] = 1;
    
    // Traverse through all lengths from 1 to n.
    for (int i=1; i<=n; i++)
    {
        // Number of subsequences with substring
        // str[0..i-1]
        dp[i] = 2*dp[i-1];
 
        // If current character has appeared
        // before, then remove all subsequences
        // ending with previous occurrence.
        if (last[str[i-1]] != -1)
            dp[i] = dp[i] - dp[last[str[i-1]]];
 
        // Mark occurrence of current character
        last[str[i-1]] = (i-1);
    }
    
    
    return dp[n]-1;
    
    
}

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Compilation time: 0.72 sec, absolute running time: 0.14 sec, cpu time: 0 sec, memory peak: 3 Mb, absolute service time: 0,87 sec