Sort row sorted matrix, C++ (gcc)

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//Title of this code
//g++  4.8.2

#include <iostream>
#include <string>
#include <vector>

std::vector<int> sortMatrix(std::vector<std::vector<int>>& matrix) {

std::vector<int> sorted;
    
    std::vector<int> positions(matrix.size());
    for (int i = 0; i < matrix.size(); ++i)
        positions[i] = 0;
        
    int pos = 0;
    while (pos != -1) {
        
        int min = 9999;
        pos = -1;

for (int i = 0; i < matrix.size(); ++i) {
            if (positions[i] < matrix[i].size() && matrix[i][positions[i]] < min) {
                min = matrix[i][positions[i]];
                pos = i;
            } 
        }
        
        if (pos != -1) {
            sorted.push_back(min);
            ++positions[pos];
        }  
    }
            
    return sorted;
}

int main()
{
    std::cout << "Hello, world!\n";
    
    std::vector<std::vector<int>> matrix = {{1,2,3},{1,4,5},{1,3,7}};
    std::vector<int> sorted;
    sorted = sortMatrix(matrix);
    
    for (int i = 0; i < sorted.size(); ++i)
        std::cout << sorted[i] << " ";
    std::cout << std::endl;
    
}

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Compilation time: 0.73 sec, absolute running time: 0.16 sec, cpu time: 0 sec, memory peak: 3 Mb, absolute service time: 0.93 sec