448. 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]

正負號標記法


遍歷數組nums，記當前元素爲index，令nums[abs(index) - 1] 爲負；


再次遍歷nums，將正數對應的下標+1返回即爲答案，因爲正數對應的元素沒有被上一步驟標記過。

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