448. Find All Numbers Disappeared in an Array

448. Find All Numbers Disappeared

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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]

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题意：

找到一个数组中没有出现的数

算法思路：

这里要注意的一个条件是1 <= a[i] <= n;

所以可以利用标志位来解决这个问题

首先定义一个相同大小的数组，把所有的值都置为-1

然后把原数组的数依次放到temp数组中，

最后遍历temp数组，-1位置上的数就是没有出现过的

代码：

package easy;import java.lang.reflect.Array;import java.util.ArrayList;import java.util.List;public class FindAllNumbersDisappearedinanArray {public List<Integer> findDisappearedNumbers(int[] nums) {List<Integer> list = new ArrayList<Integer>();int len = nums.length;int[] temp = new int[len];        for(int i=0; i<temp.length; i++){        temp[i] = -1;        }                for(int i=0; i<len; i++){        temp[nums[i]-1] = 1;        }                for(int i=0; i<len; i++){        if(temp[i] == -1){        list.add(i+1);        }        }                return list;    }}