1. class Solution {
    2. public:
    3.     int myAtoi(string str) {
    4.         int i = 0, flag = 1;
    5.         long res = 0; //默认flag = 1，正数
    6.         while (str[i] == ' ') i ++; //若str全为空格，str[i] = '\0'(最后一个i)
    7.         if (str[i] == '-') flag = -1;
    8.         if (str[i] == '-' || str[i] == '+') i ++;
    9.         for (; i < str.size() && isdigit(str[i]); i ++)  {
    10.             res = res * 10 + (str[i] - '0');
    11.             if (res >= INT_MAX && flag == 1) return  INT_MAX;
    12.             if (res > INT_MAX && flag == -1) return  INT_MIN;
    13.         } 
    14.         return flag * res;
    15.     }
    16. };

    状态机

    1. class Automaton {
    2.     string state = "start";
    3.     unordered_map<string, vector<string>> table = {
    4.         {"start", {"start", "signed", "in_number", "end"}},
    5.         {"signed", {"end", "end", "in_number", "end"}},
    6.         {"in_number", {"end", "end", "in_number", "end"}},
    7.         {"end", {"end", "end", "end", "end"}}
    8.     };
    10.     int get_col(char c) {
    11.         if (isspace(c)) return 0;
    12.         if (c == '+' or c == '-') return 1;
    13.         if (isdigit(c)) return 2;
    14.         return 3;
    15.     }
    16. public:
    17.     int sign = 1;
    18.     long long ans = 0;
    20.     void get(char c) {
    21.         state = table[state][get_col(c)];
    22.         if (state == "in_number") {
    23.             ans = ans * 10 + c - '0';
    24.             ans = sign == 1 ? min(ans, (long long)INT_MAX) : min(ans, -(long long)INT_MIN);
    25.         }
    26.         else if (state == "signed")
    27.             sign = c == '+' ? 1 : -1;
    28.     }
    29. };
    31. class Solution {
    32. public:
    33.     int myAtoi(string str) {
    34.         Automaton automaton;
    35.         for (char c : str)
    36.             automaton.get(c);
    37.         return automaton.sign * automaton.ans;
    38.     }
    39. };