2021-07-24 把数组排成最小的数

全排列，使用treeSet，长度一定的情况下，自然序排第一个的就是最小的。

public String PrintMinNumber(int [] numbers) {
        
        if(null == numbers || numbers.length == 0){
            return "";
        }
        TreeSet<String> treeSet = new TreeSet();
        perm(numbers,0,treeSet);

        return treeSet.first();
    }

    public void perm(int[] nums,int index,TreeSet<String> result){
        if(index == nums.length-1){
            StringBuilder temp = new StringBuilder();
            for (int i = 0; i < nums.length; i++) {
                temp.append(nums[i]);
            }
            result.add(new String(temp));
            return;
        }
        for(int i = index;i<nums.length;i++){
            if(i == index || nums[index] != nums[i]){
                swap(nums,i,index);
                perm(nums,index+1,result);
                swap(nums,i,index);
            }
        }
    }

    public void swap(int[] nums,int i,int j){
        if( i == j){
            return;
        }
        nums[i] = nums[i] + nums[j];
        nums[j] = nums[i] - nums[j];
        nums[i] = nums[i] - nums[j];
    }