solutions to problems in Project Euler Source files function function name and file n Choose k nchoosek(n,k) GCD of two numbers gcd(a,b) Factorial factorial(n) Solutions ID Problem Solution 25 1000-digit Fibonacci Number string addition of large numbers. solution 20 Factorial Digit Sum Uses the same logic as power digit sum. Solution 16 Power Digit Sum 016_power_digit_sum.cpp 15 Lattice Paths use double when dealing with division Solution 14 Longest Collatz Sequence 014_CollatzSequence.cpp 13 Large Sum Solution - 013_largesum.cpp 12 Highly Divisible Triangular Number Solution - 012_Highlydivisibletriangularnumber.cpp 11 Largest Product in a Grid Solution - 011_productinagrid.cpp 10 Summation of Primes Solution - 010_sum_primes.cpp 9 Special Pythagorean Triplet Solution - 009_PythagoreanTriple.cpp 8 Largest Product in a Series Solution - 008_Largest_product_series.cpp 7 10001st Prime Solution - 007_nth_prime.cpp 6 Sum Square Difference Solution - 006_SumSquareDifference.cpp 5 Smallest Multiple Solution - 005_smallest_multiple.cpp 4 Largest Palindrome Product Solution - 004_Largestpalindromeproduct.cpp 3 Largest Prime Factor Solution - 003_primefactor.cpp 2 Even Fibonacci Numbers Solution - 002_even_fibonacci.cpp 1 Multiples of 3 or 5 Solution - 001_Multiplesof3and5.cpp