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]

解题思路：

用unordered_map扫描一遍。

 

class Solution {
public:
vector<int> findDuplicates(vector<int>& nums) {

unordered_map<int,int> store;
vector<int> duplicate;
for(int i=0;i<nums.size();i++){

if(store.count(nums[i]) == 1) {
duplicate.push_back(nums[i]);
}
else{
store[nums[i]]++;
}

}

return duplicate;
}
};