Find All Duplicates in an Array 找出数组中所有重复项

//solution1
/**
//references:https://www.cnblogs.com/grandyang/p/6209746.html
  */
class Solution1{
public:
    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;
    }
};

class Solution2{
public:
    vector<int> findDuplicates(vector<int>& nums){
        vector<int> res;
        for(int i = 0; i < nums.size(); i++){
            if(nums[i] != nums[nums[i]-1]){
                swap(nums[i],nums[nums[i]-1]);
                --i;
            }
        }

        for(int i = 0; i < nums.size(); i++){
            if(nums[i] != i+1) res.push_back(nums[i]);
        }
        return res;
    }
};


class Solution3{
public:
    vector<int> findDuplicates(vector<int>& nums){
        vector<int> res;
        int n = nums.size();
        for(int i = 0; i < n; ++i){
            nums[(nums[i]-1)%n] += n;
        }

        for(int i = 0; i < n; ++i){
            if(nums[i] > 2*n) res.push_back(i+1);
        }
        return res;
    }
};