哈希表:
class Solution {
public:
char firstUniqChar(string s) {
int len = s.size();
unordered_map<char, int> res;
for (auto c : s) {
res[c] += 1;
}
for (auto c : s) {
if (res[c] == 1) {
return c;
}
}
return ' ';
}
};