Unordered map in c++

Question: Given a non-empty array of integers, every element appears twice except for one. Find that single one.

Note:

Your algorithm should have a linear runtime complexity. Could you implement it without using extra memory?

Example 1:

Input: [2,2,1] Output: 1 Example 2:

Input: [4,1,2,1,2] Output: 4

I implemented this problem in C++ using unordered map. My solution:

class Solution {
 public:
  int singleNumber(vector<int>& nums) {
    std::unordered_map<int, int> umap;

    for (auto i = nums.begin(); i != nums.end(); i++) {
      umap[*i] = umap[*i] + 1;
    }

    for (auto i = umap.begin(); i != umap.end(); i++) {
      if (umap[*i] == 1) {
        return *i;
      }
    } 
  }
};

But unfortunately, it does not work. I get this error while compiling

Line 16: Char 17: fatal error: no viable overloaded operator[] for type 'std::unordered_map' if (umap[*i] == 1) { ~~~~^~~ /usr/bin/../lib/gcc/x86_64-linux-gnu/8/../../../../include/c++/8/bits/unordered_map.h:973:7: note: candidate function not viable: no known conversion from 'std::pair' to 'const std::unordered_map, std::equal_to, std::allocator > >::key_type' (aka 'const int') for 1st argument operator[](const key_type& __k) ^ /usr/bin/../lib/gcc/x86_64-linux-gnu/8/../../../../include/c++/8/bits/unordered_map.h:977:7: note: candidate function not viable: no known conversion from 'std::pair' to 'std::unordered_map, std::equal_to, std::allocator > >::key_type' (aka 'int') for 1st argument operator[](key_type&& __k) ^ 1 error generated.

I could not understand the error. Can anyone explain to me.

Thank you in advance.