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Find all palindromes made from the product of two 4-digit numbers
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#include <iostream> | |
#include <iomanip> | |
#include <unordered_set> | |
#include <vector> | |
#include <fstream> | |
using Type = uint32_t; | |
using Magic = std::vector<Type>; | |
using Numbers = std::unordered_set<Type>; | |
Numbers palindromes; | |
Magic doMagic(int length) | |
{ | |
Magic magic; | |
magic.emplace_back(0); | |
magic.emplace_back(1); | |
Type x = 1; | |
for (int i=1; i<length; ++i) { | |
x *= 10; | |
magic.emplace_back(x + 1); | |
} | |
return magic; | |
} | |
void calculate(Type x, int length, Type base) | |
{ | |
const static auto magic = doMagic(10); | |
for (int i=0; i<=9; ++i) { | |
if (length > 2) { | |
calculate(x, length - 2, base * 10); | |
} else { | |
if (x % 10) palindromes.emplace(x); | |
} | |
x += (base * magic[length]); | |
} | |
} | |
int main(int argc, char** argv) | |
{ | |
Type MIN=1000, MAX=9999; | |
calculate(0, 7, 1); | |
calculate(0, 8, 1); | |
std::ofstream os("output.txt"); | |
for (Type a=MIN; a<=MAX; ++a) { | |
if (a % 10 == 0) continue; | |
for (Type b=a; b<=MAX; ++b) { | |
Type x = a * b; | |
Type l = x / 10000000; | |
if (l == 0) { | |
l = x / 1000000; | |
} | |
if (l == x % 10 && palindromes.count(x)) { | |
os << a << ' ' << b << ' ' << x << std::endl; | |
} | |
} | |
} | |
return 0; | |
} |
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