448. Find All Numbers Disappeared in an Array.java

import java.util.List;

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