字符串转换整数 Atoi

  1. class Solution {
  2. public:
  3.    int myAtoi(string str) {
  4.        int res = 0;
  5.        int i = 0;
  6.        int flag = 1;
  7.        while(str[i] == '') {
  8.            i++;
  9.        }
  10.        if(str[i] == '-') {
  11.            flag = -1;
  12.        }
  13.        if(str[i] == '+' || str[i] == '-') {
  14.            i++;
  15.        }
  16.        while(i < str.size() && isdigit(str[i])) {
  17.            int r = str[i] - '0';
  18.            if(res > INT_MAX / 10 || (res == INT_MAX / 10 && r > 7)) {
  19.                return flag > 0 ? INT_MAX : INT_MIN;
  20.            }
  21.            res = res * 10 + r;
  22.            i++;
  23.        }
  24.        return flag > 0 ? res : -res;
  25.    }
  26. };

自动机

fig1.png

  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. };