387 字符串中的第一个唯一字符

https://leetcode.cn/leetbook/read/top-interview-questions-easy/xnx13t/

https://leetcode.cn/problems/reverse-integer/description/

cost: 6m48s

class Solution {
public:
    int firstUniqChar(string s) {
        unordered_map<int, int> m;
        for (int i = 0; i < s.size(); i++) {
            char ch = s[i];
            if (m.count(ch)) {
                m[ch] = -1;
            } else {
                m[ch] = i;
            }
        }
        int r = -1;
        for (const auto [_, pos] : m) {
            if (pos != -1) {
                if (r == -1 || r > pos) {
                    r = pos;
                }
            }
        }
        return r;
    }
};

• 时间复杂度：O(n)
• 空间复杂度：O(∣Σ∣)，其中 Σ 是字符集