## 题目描述

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]``````

## 解法一

### 思路

`[4,3,2,7,8,2,3,1]`演示一遍过程：

``````[ 4,  3,  2,  7,  8,  2,  3,  1] -> [ 4,  3,  2, -7,  8,  2,  3,  1]
[ 4,  3,  2, -7,  8,  2,  3,  1] -> [ 4,  3, -2, -7,  8,  2,  3,  1]
[ 4,  3, -2, -7,  8,  2,  3,  1] -> [ 4, -3, -2, -7,  8,  2,  3,  1]
[ 4, -3, -2, -7,  8,  2,  3,  1] -> [ 4, -3, -2, -7,  8,  2, -3,  1]
[ 4, -3, -2, -7,  8,  2, -3,  1] -> [ 4, -3, -2, -7,  8,  2, -3, -1]
[ 4, -3, -2, -7,  8,  2, -3,  1] -> [ 4, -3, -2, -7,  8,  2, -3, -1]
[ 4, -3, -2, -7,  8,  2, -3,  1] -> [ 4, -3, -2, -7,  8,  2, -3, -1]
[ 4, -3, -2, -7,  8,  2, -3,  1] -> [-4, -3, -2, -7,  8,  2, -3, -1]

output: [5, 6]``````

### Python

``````class Solution(object):
def findDisappearedNumbers(self, nums):
"""
:type nums: List[int]
:rtype: List[int]
"""
for num in nums:
index = num if num > 0 else -num
index -= 1
if nums[index] > 0:
nums[index] = -nums[index]
result = []
for (index, num) in enumerate(nums):
if num > 0:
result.append(index + 1)
return result``````

### Java

``````public class Solution {
public List<Integer> findDisappearedNumbers(int[] nums) {
for (int num : nums) {
int index = num > 0 ? num : -num;
index--;
if (nums[index] > 0) {
nums[index] = -nums[index];
}
}
List<Integer> result = new ArrayList<>();
for (int i = 0; i < nums.length; i++) {
if (nums[i] > 0) {
}
}
return result;
}
}``````

### C++

``````class Solution {
public:
vector<int> findDisappearedNumbers(vector<int> &nums) {
for (auto num : nums) {
int index = num > 0 ? num : -num;
index--;
if (nums[index] > 0) {
nums[index] = -nums[index];
}
}
vector<int> result;
for (int i = 0; i < nums.size(); i++) {
if (nums[i] > 0) {
result.push_back(i + 1);
}
}
return result;
}
};``````