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/*
Challenge 245
Task 2: Largest of Three
Submitted by: Mohammad S Anwar
You are given an array of integers >= 0.
Write a script to return the largest number formed by concatenating some of
the given integers in any order which is also multiple of 3. Return -1 if
none found.
Example 1
Input: @ints = (8, 1, 9)
Output: 981
981 % 3 == 0
Example 2
Input: @ints = (8, 6, 7, 1, 0)
Output: 8760
Example 3
Input: @ints = (1)
Output: -1
*/
#include <iostream>
#include <vector>
using namespace std;
int calc_largest(int prefix, const vector<int>& nums) {
int largest = -1;
// check current prefix
if (prefix != 0 && prefix % 3 == 0 && prefix > largest)
largest = prefix;
// check all compinations of each number in nums
vector<int> remain;
for (size_t i = 0; i < nums.size(); i++) {
// create list of numbers excluding n
int n = nums[i];
remain = nums;
remain.erase(remain.begin() + i);
int this_largest = calc_largest(prefix * 10 + n, remain);
if (this_largest > largest)
largest = this_largest;
}
return largest;
}
int main(int argc, char* argv[]) {
if (argc < 2) {
cerr << "Usage: ch-2 n n n..." << endl;
exit(EXIT_FAILURE);
}
vector<int> nums;
for (int i = 1; i < argc; i++)
nums.push_back(atoi(argv[i]));
int largest = calc_largest(0, nums);
cout << largest << endl;
}
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