Find All Numbers Disappeared in an Array

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]

思路：将数字与下标对应起来，若不匹配的说明当前值缺失;

JAVA CODE

class Solution {
    public List<Integer> findDisappearedNumbers(int[] nums) {
        List<Integer> list = new ArrayList<>();
        int[] mm = new int[nums.length];
        for(int i = 0; i < nums.length; i++){
            mm[nums[i]-1] = nums[i];
        }
        for(int i = 0; i < nums.length; i++){
            if(mm[i]-1!=i)
                list.add(new Integer(i+1));
        }
        return list;
    }
}