LetCode 8. 字符串转整数 (atoi)

class Solution {
public:
    int myAtoi(string str) {
        int len = str.length();
        int index = 0;
        long long num = 0;
        bool start = false;
        int sym = 1;
        while (index < len) {
            if (start == false && str[index] == ' ') {
                index++;
                continue;
            }
            if ((start == false && (str[index] == '-' || str[index] == '+')) || ('0' <= str[index] && str[index] <= '9')) {
                if (str[index] == '-')
                    sym = -1;
                if ('0' <= str[index] && str[index] <= '9') {
                    if (!start) {
                        num = str[index] - 48;
                    }
                    else {
                        num *= 10;
                        num += (str[index] - 48);
                        if (num * sym < 0 && num > 2147483648)
                            return -2147483648;
                        if (num * sym > 0 && num > 2147483647)
                            return 2147483647;
                    }
                }
                if (start == false)
                    start = true;
            }
            else {
                if (start)
                    break;
                else
                    return 0;
            }
            index++;
        }
        return num * sym;
    }
};