字符类型相关操作和数值类型转换

    // atoi
    class Solution {
    public:
    int myAtoi(string str) {
        int res = 0;
        int i = 0;
        int flag = 1;
        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 : -