448、Find All Numbers Disappeared in an Array

一、问题：

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]

二、解题

class Solution {
public:
/*
不管一个元素出现一次还是两次，只要出现它对应的位置就会被取负。
当某个元素不出现的时候，该元素对应的位置始终访问不到，所以还是正值，
通过这种方法我们就可以找到哪些元素没有出现。

*/
vector<int> findDisappearedNumbers(vector<int>& nums) {
int len = nums.size();
for (int i = 0; i<len; i++) {
int m = abs(nums[i]) - 1; // index start from 0
nums[m] = nums[m]>0 ? -nums[m] : nums[m];
}
vector<int> res;
for (int i = 0; i<len; i++) {
if (nums[i] > 0) res.push_back(i + 1);
}
return res;
}
};

时间复杂度O(n)。