题目：

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]

链接：

3/25/2017

performance 14% 35ms

1 public class Solution { 2     public List
     
       findDisappearedNumbers(int[] nums) { 3         List
      
        ret = new ArrayList
       
        (); 4         if (nums.length == 0) return ret; 5  6         for (int i = 0; i < nums.length; i++) { 7             ret.add(0); 8         } 9         for (int i = 0; i < nums.length; i++) {10             ret.set(nums[i] - 1, ret.get(nums[i] - 1) + 1);11         }12         int index = 0;13         for (int i = 0; i < nums.length; i++) {14             if (ret.get(i) == 0) {15                 ret.set(index, i + 1);16                 index++;17             }18         }19         return ret.subList(0, index);20     }21 }
       
      
     

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