Given an array of integers where 1 ≤ a[i] ≤ n (n = size of array), some elements appear twice and others appear once.

Find all the elements of [1, n] inclusive that do not appear in this array.

Could you do it without extra space and in O(n) runtime? You may assume the returned list does not count as extra space.

Example:

Input:

[4,3,2,7,8,2,3,1]

Output:

[5,6]

这道题要求找出数组中消失的数据，我的解题思路是通过一个伪哈希表记录存在的数据，然后再遍历显示没有统计的。

    public static List<Integer> findDisappearedNumbers(int[] nums) {
        ArrayList<Integer> arrayList = new ArrayList<Integer>();
        int[] ary = new int[nums.length+1];
        for (int i = 0; i < nums.length; i++) {
            ary[nums[i]]++;
        }
        for (int i = 1; i < ary.length; i++) {
            if (ary[i] == 0) {
                arrayList.add(i);
            }
        }
        return arrayList;
    }

更好的实现方案：http://www.cnblogs.com/grandyang/p/6222149.html

[LeetCode] 448.Find All Numbers Disappeared in an Array