题目：

题解：

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