Problem 3

The prime factors of 13195 are 5, 7, 13 and 29.

What is the largest prime factor of the number 600851475143 ?

C++:

#include 
     
      using namespace std;long maxfact(long n){    long ret=2L, i;    while(n%2 == 0)        n /= 2;    for(i=3; i*i<=n; i+=2) {        if(n%i == 0) {            ret = i;            n /= i;            while(n%i == 0)                n /= i;        }    }    return (n==1)?ret:n;}int main(){    long n;    while(cin >> n) {        cout << maxfact(n) << endl;    }    return 0;}
     

Run results:

13195     

29

600851475143

6857

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