Maximum number formed from array list of integers

Question

Problem: Given a list of non negative integers, arrange them such that they form the largest number.

My approach:

public class Solution implements Comparator<Integer>{
    public String largestNumber(final List<Integer> A) {
        List<Integer> B = A;
        Collections.sort(B);
        String ans = "";
        for(int i=B.size()-1;i>=0;i--)
            ans = ans+Integer.toString(B.get(i));
        return ans;
    }
    public int compare(Integer a,Integer b){
        String as = Integer.toString(a);
        String bs = Integer.toString(b);
        String fe = as+bs;
        String se = bs+as;
        return (fe.compareTo(se));
    }
}

Problems arising: For A = [3, 30, 34, 5, 9] the output being shown is 3430953 but expected output is 9534330.

From what I can see array list is sorted normally without using custom made compare() method. Why is this happening?

Answer 1

Where you have this:

Collections.sort(B);

you are sorting with the natural order of the elements because you don't specify a comparator.

Since you are inside the Solution class, and it implements Comparator<Integer> , you probably mean:

Collections.sort(B, this);

so you will actually use the ordering that you have defined.

Answer 2

Python Version :

import functools
from functools import cmp_to_key

num=[3, 30, 34, 5, 9]

def compare(n1, n2):
    if n1 + n2 > n2 + n1:
        return 1
    elif n1 + n2 < n2 + n1:
        return -1
    else:
        return 0

num_str = [str(n) for n in num]
res = ""
print(num_str)

res=""
for n in sorted(num_str,key=functools.cmp_to_key(compare),reverse=True) :
            res += n

print(res)