Largest Number

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

For example, given [3, 30, 34, 5, 9], the largest formed number is 9534330.

Note: The result may be very large, so you need to return a string instead of an integer.

写个comparator 来排序，排序的依据是：对任意两个s1，s2，如果s1+s2 > s2+s1，则s1应该排在前面。例如： s1=5, s2=34, 534>345, 则5应该排在前面。再如，s1=30， s2=34, 3034 < 3430，那么34应该排在前面。

要注意写个comparator，与Merge K Sorted Lists 里面用min heap时用到的comparator一样。

public class Solution {
    public String largestNumber(int[] num) {
        String[] numStrings = new String[num.length];
        for(int i = 0; i < num.length; i++) {
            numStrings[i] = String.valueOf(num[i]);
        }
        Comparator<String> comp = new Comparator<String>() {
            public int compare(String s1, String s2) {
                return -(s1 + s2).compareTo(s2 + s1);
            }
        };
        Arrays.sort(numStrings, comp);
        StringBuilder sb = new StringBuilder();
        for(String s : numStrings) {
            if(!(s.equals("0") && sb.length() == 0)) {
                sb.append(s);
            }
        }
        return sb.length() > 0 ? sb.toString() : "0";
    }
}