Sword Finger Offer Interview Question 50. The first character that appears only once [Simple]-unordered_map

My solution:

Use unordered_map and vector

Note the difference between unordered_map and map.

The elements in the map are stored according to the binary search tree, and the middle-order traversal can be used to traverse the key values ​​from small to large. (A red-black tree
 is implemented internally ) unordered_map internally implements a hash table (also known as a hash table), which accesses records by mapping key code values ​​to a location in the Hash table. The time complexity of the search can reach O (1) , Which is widely used in massive data processing). The arrangement order of its elements is disordered. (It is unordered, not in the order of insertion. I remembered wrong at first, thinking that this question was done in the order of insertion, the first example can't get through ... I have been searching for a long time)

PS: I do n’t know about the Red and Black Trees yet, so I need to learn! Learn!

class Solution {
public:
    char firstUniqChar(string s) {
        if(s.empty())   return ' ';
        unordered_map<char,int> m;
        vector<char> vec;
        for(char c:s){
            if(m.count(c))  m[c]++;
            else{
                vec.push_back(c);
                m[c]=1;
            }
        }
        for(int i=0;i<vec.size();i++){
            if(m[vec[i]]==1)    return vec[i];
        }
        return ' ';
    }
};