448. Find All Numbers Disappeare

LeetCode Find All Numbers Disappeared in an Array【Easy】

• 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]

解决

题目主要是找出给定数组中所有不出现的数字，解决该为题主要是找出nums[i]与nums[|nums[i]|-1]其中的关系，我们这边采用取负方法进行求解。代码及部分注释如下

代码

  /**
   *  找出数组中的值与其值应该在下标的位置关系
   *  nums[i]与nums[|nums[i]|-1]
   *  取负值 如果为正则取该值对应的下标+1
   * @param nums
   * @return
   */
  public List<Integer> findDisappearedNumbers(int[] nums) {

      List<Integer> res = new ArrayList<>();
      //判断值关系
      for(int i=0;i<nums.length;i++){
          int index = Math.abs(nums[i])-1;
          nums[index]=(nums[index]>0)?(-nums[index]):nums[index];
      }
      //取出数组中值为正的下标+1
      for(int i=0;i<nums.length;i++){
          if(nums[i]>0){
              res.add(i+1);
          }
      }
      return res;
  }