First Unique Character in a String — C++

// Time:  O(n)
// Space: O(n)

// One-pass solution.
class Solution {
public:
    int firstUniqChar(string s) {
        using IT = list<int>::iterator;

    	list<int> candidates;
    	unordered_map<char, IT> lookup;
    	for (int i = 0; i < s.length(); ++i) {
    		const auto c = s[i];
    		if (lookup.count(c)) {
    			if (lookup[c] != candidates.end()) {
    				candidates.erase(lookup[c]);
    			}
    			lookup[c] = candidates.end();
    		} else {
    			lookup[c] = candidates.emplace(candidates.end(), i);
    		}
    	}
    	return candidates.empty() ? -1 : candidates.front();
    }
};