https://leetcode.com/problems/missing-number/description/
解题思路：
- 先排序，然后判断nums[i] == i
代码：
class Solution {
public int missingNumber(int[] nums) {
if(nums == null) return 0;
Arrays.sort(nums);
int len = nums.length;
for(int i = 0; i < len; i++){
if(nums[i] != i)
return i;
}
return len;
}
}
