[leetcode-448]Find All Numbers Disappeared in an Array

Problem Description

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]

Solution

For an array whoes elements are in range of 1 to n, considerate to use the element as the index to identify the missed or duplicated elements.
The key point is locate and mark, with the following logic:

array[array[i] - 1] *= -1; // where i = 0, 1, 2, 3... , n-1

code:

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