题目描述：

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:

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

Output:
[2,3]

跟LeetCode 448. Find All Numbers Disappeared in an Array类似，只不过这次是寻找出现过两次的数。我用0（输入数据中不会出现）来表示该数已经记录过来防止重复计数。

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