最后的结果可能很大，所以我们返回一个字符串来代替这个整数。

public class Solution {

    /**

     *@param num: A list of non negative integers

     *@return: A string

     */

    public String largestNumber(int[] num) {

        // write your code here

        ArrayList<String> list = new ArrayList<String>();

        for(int i:num)

            list.add(i+"");

        Collections.sort(list,new Comparator<String>(){

            public int compare(String left,String right){

                String leftright = left + right;

                String rightleft = right + left;

                return leftright.compareTo(rightleft);

            }

        });

        String result="";

        for(int i =list.size()-1;i>=0;i--)

            result+= list.get(i);

        // if(result.equals("00") ||result.equals("0000")||result.equals("00000"))

        //     return "0";

        // 去除左边无效的 0

        int i = 0;

        while(i< result.length() && result.charAt(i) =='0')

            i++;

        if(i==result.length())

            return "0";

        return result.substring(i);

    }

}

public class Solution {

    /**

     *@param num: A list of non negative integers

     *@return: A string

     */

    public String largestNumber(int[] num) {

        // write your code here

        String result = "";

        quickSort(num,0,num.length - 1);

        for(int i = 0;i<num.length;i++)

            result += num[i];

        // 去除左边无效的 0

        int i = 0;

        while(i< result.length() && result.charAt(i) =='0')

            i++;

        if(i==result.length())

            return "0";

        return result.substring(i);

    }

    // 可以理解为逆序排序，排序后直接链接起来就是答案

    public void quickSort(int[] num,int low,int high){

        if(low>high)

            return;

        // for(int kk:num)

        //     System.out.print(kk+"  ");

        // System.out.println();

        int i= low;

        int j= high;

        int x = num[i];

        while(i<j){

            while(i<j && compare(x,num[j]))

                j--;

            if(i<j){

                num[i] = num[j];

                i++;

            }

            while(i<j && compare(num[i],x))

                i++;

            if(i<j){

                num[j] = num[i];

                j--;

            }

        }

        num[i] = x;

        quickSort(num,low,i-1);

        quickSort(num,i+1,high);

    }

    // AB > BA 说明A应该在前面，B应该在后面

    public boolean compare(int A,int B){

            String left = A+"";

            String right =B+"";

            String leftright = left + right;

            String rightleft = right + left;

            return leftright.compareTo(rightleft) >0;

    }

}