Leecode8 代码：如下，很简单，一看就明白的。
int Leecode8_myAtoi(std::string str) { int res= 0; int i = 0; int flag = 0; while (str[i] == ' ') i++; if (str[i] == '-') flag = -1; if (str[i] == '+' || str[i] == '-') i++; while (i < str.size() && isdigit(str[i])) { int r = str[i] - '0'; if (res > INT_MAX / 10 || (res == INT_MAX / 10 && r > 7)) return flag >= 0 ? INT_MAX : INT_MIN; res = res * 10 + r; i++; } return flag >= 0 ? res : -res; }