Split the string into prime segments

#include <bits/stdc++.h>
using namespace std;
/*
Logic:
->Using Sieve of erathosness,stored all the prime numbers in string
  data type in a map.
 ->dp array will store the minimum segment possible ie. dp[i]=minimum
   segment possible till ith index.
 ->It's must to have a prime from 0th index to ith index where i<=5
   otherwise that part will not be prime and answer will be -1.
 ->let's say there is prime number form lth to rth index,then go
   from (r+1)th index to (r+i)th index where i<=6 and check if that
   if prime then update the dp array.
*/
void sieve(unordered_map<string,int> &primes){
    bool prime[1000001];
    memset(prime, true, sizeof(prime));
    prime[0] = prime[1] = false;
    for (int i = 2; i * i <= 1000000; i++) {
        if (prime[i] == true) {
            for (int j = i * i; j <= 1000000; j += i)
                prime[j] = false;
        }
    }
    for (int i = 2; i <= 1000000; i++) {
        if (prime[i] == true){
            primes[to_string(i)]=1;
        }
    }
}
int splitIntoPrimes(string number){
    int n= number.length();
    int dp[n + 1];
    memset(dp, -1, sizeof(dp));
    unordered_map<string,int>primes;
    sieve(primes);
    for (int i = 1; i <= n; i++) {
        if (i <= 6 && primes[number.substr(0, i)]){
            dp[i] = 1;
        }
        if (dp[i]!=-1) {
            for (int j = 1; j <= 6 && i + j <= n; j++) {
                if (primes[number.substr(i, j)]){
                    if (dp[i + j]==-1)
                        dp[i + j] = 1 + dp[i];
                    else
                        dp[i + j] = min(dp[i + j],1 + dp[i]);
                }
            }
        }
    }
    return dp[n];
}
int main(){
    cout << splitIntoPrimes("13499315") << "\n";
    return 0;
}