【66】442. Find All Duplicates in an Array

442. Find All Duplicates in an Array

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• Total Accepted: 15294
• Total Submissions: 30028
• Difficulty: Medium
• Contributors: shen5630

 

Given an array of integers, 1 ≤ a[i] ≤ n (n = size of array), some elements appear twice and others appear once.

Find all the elements that appear twice in this array.

Could you do it without extra space and in O(n) runtime?

 

Example:

Input:
[4,3,2,7,8,2,3,1]

Output:
[2,3]

class Solution {
public:
//因为n[i]是在1到n之间，如果把n[i] - 1看成index的话，把其对i应的n[n[i]-1]换成负数，如果之前已经是负数，说明换过了（n[i] + 1重复）。
    vector<int> findDuplicates(vector<int>& nums) {
        vector<int> res;
        for(int i = 0; i < nums.size(); i++){
            int idx = abs(nums[i]) - 1;
            if(nums[idx] < 0){
                res.push_back(idx + 1);
            }
            nums[idx] = -nums[idx];
        }
        return res;
    }
};