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find-all-numbers-disappeared-in-an-array

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Find All Numbers Disappeared in an Array – LeetCode 448

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]

Answer

Original

Code

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

思路

选择使用数字正负来标记该位是否存在，不错的想法。时间复杂度$O(n)$，空间复杂度$O(1)$。
耗时$135$ ms，排名$49.03\%$

Better

思路

还没看到更好的思路。