How to find the largest number formed from an array of numbers

Problem statement

Assume we are given an array of non-negative integers, and we need to arrange the numbers in such a way that they form the largest possible number.

#include <iostream>
#include <algorithm>
#include <string>
using namespace std;
bool compareNumbers(int a, int b)
{
    string num1 = to_string(a);
    string num2 = to_string(b);
    return (num1 + num2) > (num2 + num1);
}
int findMaxNum(int arr[], int n)
{
    sort(arr, arr + n, compareNumbers);
    string largestNumStr = "";
    for (int i = 0; i < n; i++)
    {
        largestNumStr += to_string(arr[i]);
    }
    return stoi(largestNumStr);
}
int main()
{
    int arr[] = {10, 7, 9, 2, 3};
    int n = sizeof(arr) / sizeof(arr[0]);
    int largestNumber = findMaxNum(arr, n);
    cout << "Largest Number: " << largestNumber << endl;
    return 0;
}

Code example for largest number formation

Explanation

Now, let's take a closer look at the C++ code and understand its workings in depth.

Lines 7-12: compareNumbers function takes two integers as input arguments and compares them by converting them to strings and concatenating them.
Lines 9-10: Converts the integer a and integer b to a string using the to_string function from the string library.
Line 11: Compares the concatenation of num1 and num2 with the concatenation of num2 and num1 and returns true if the first concatenation is greater than the second concatenation, and false otherwise.
Line 15: findMaxNum takes an array of integers arr and its size n as input.
Line 17: Sorts the array arr in descending order using the compareNumbers custom comparison function.
Line 19: Initializes an empty string named largestNumStr.
Line 22: This line converts each element of the array to a string and appends it to the largestNumStr string.
Line 25: This line converts the largestNumStr string to an integer using the stoi function and returns it.

Sample input	Output
[10, 7, 9, 2, 3]	973210
[4, 6, 8, 2, 3]	86432