Largest Number

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

Have you met this question in a real interview? Yes
Example
Given [1, 20, 23, 4, 8], the largest formed number is 8423201.

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

public class Solution {  
    public String largestNumber(int[] num) {  
        int n = num.length;  
        if (n < 1) return "";  

        //把数组转化为字符串数组  
        String[] strs = new String[n];  
        for (int i = 0; i < n; i++) {  
            strs[i] = String.valueOf(num[i]);  
        }  

        //按照数字在结果中的位置排序  
        Arrays.sort(strs, new Cmp());  

        //把所有字符串拼接在一起  
        String ans = "";  
        for (int i = n - 1; i >= 0; i--) {  
            ans = ans.concat(strs[i]);  
        }  

        //去掉数字开头的0，如输入[0, 0]  
        int i = 0;  
        while (i < n && ans.charAt(i) == '0') {  
            i++;  
        }  
        if (i == n) return "0";  

        return ans.substring(i);  
    }  
}  

//比较方法很简单：如果[a在前b在后]组成的结果大于[b在前a在后]组成的结果，那么a>b。  
class Cmp implements Comparator<String>{  
    @Override  
    public int compare(String a, String b) {  
       String   ab = a.concat(b);
        String  ba = b.concat(a);

        return (ab).compareTo(ba);
    }  
}