Find the largest multiple of 3, given an array of n digits

#include <stdio.h>
#include <vector>
#include <algorithm>

using namespace std;
typedef vector<int> Container;

/*
 * Algorithm:
 *    a . Sort the array
 *    b . find the sum of the array
 *    c.  find rem = sum % 3
 *      c1. if rem == 0 return 0;
 *          else
 *            find first digit whose remainder == rem
 *            if found
 *              erase it from the array and return 0
 *            else
 *              find first two digits whose remainder != 0 and != rem
 *                if found
 *                  erase them and return 0
 *                else
 *                  return -1  
 */

/**
 *  Return:
 *    0 success
 *    -1 failure
 */

int largestDivBy3(Container &v) {

    sort(v.begin(), v.end());
    
    int s = accumulate(v.begin(), v.end(), 0),
        rem = s % 3, 
        tmp;

    if (rem == 0) return 0;
    
    int pos1 = -1,
        pos2 = -1;

    for (auto i = v.begin(); i != v.end(); ++i ) {
        tmp = *i % 3;
        if (tmp == rem) {
            v.erase(i);
            return 0;
        } else if(tmp) {
           if (pos1 == -1) {
                pos1 = i - v.begin();
           } else if(pos2 == -1) {
                pos2 = i - v.begin();
           }
        }
    }

    if (pos2 != -1) {
        v.erase(v.begin() + pos1);
        v.erase(v.begin() + pos2 - 1);
        return 0;
    }
    return -1;
}


int main() {
    Container v = {9, 7, 6,  6, 6 };
    
    int n = largestDivBy3(v);

    if (n == 0) {
        for(auto i  = v.rbegin(); i < v.rend(); ++i) {
            printf("%d", *i);
        }

        printf("\n");
    } else {
        printf("Not possible");
    }
}