448. Find All Numbers Disappeared in an Array

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]

我们把所有出现过的数的位置设为负值，最后没有出现过的位置就是正值。用三目运算符保证我们不会覆盖未遍历的值，仅仅将其设置为负值。

class Solution {public:    vector<int> findDisappearedNumbers(vector<int>& nums) {        for(int i=0; i<nums.size(); ++i){            int m = abs(nums[i]) - 1;            nums[m] = nums[m] > 0 ? -nums[m] : nums[m];        }         vector<int> res;           for(int i=0; i<nums.size(); ++i){            if(nums[i] > 0)                  res.push_back(i+1);        }        return res;    }};