leetcode448 Find All Numbers Disappeared In An Array Java

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]

思路很简单，两次循环。第一次，把nums数组中的值做一定处理当做下标，标记下标（下标其实就是nums数组中的值，出现过的就会被标记。不知道说清楚了没，比如下面这个解法就是把在nums中出现过的下标的值都变成负数了）。 第二次，把标记的下标add到List中。

public List<Integer> findDisappearedNumbers(int[] nums) {
List<Integer> result = new ArrayList<>();
for(int i=0; i<nums.length; i++) {
int value = Math.abs(nums[i]) -1;
if(nums[value] > 0) {
nums[value] = -nums[value];
}
}

for(int i=0; i<nums.length;i++) {
if(nums[i] > 0) result.add(i+1);
}
return result;
}