leetcode刷題系列--442. Find All Duplicates in an Array

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]

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class Solution {
public:
    vector<int> findDuplicates(vector<int>& nums) {
        int len = nums.size();
        vector<int> res;
        for(int i = 0; i < len; ++i)
        {
            if(nums[i] == i + 1)
                continue;
            else
            {
                while(nums[i] != i + 1 && nums[nums[i] - 1] != nums[i])
                {
                    int tmp = nums[i];
                    nums[i] = nums[tmp - 1];
                    nums[tmp - 1] = tmp;
                }
            }
        }
        
        for(int i = 0; i < len; ++i)
        {
            if(nums[i] != i + 1)
                res.push_back(nums[i]);
        }
        return res;
    }
};

一遍遍的交換，直到所有的數字都在自己所在的位置，當出現不在自己位置的數字時，說明該數字是有重複的。