[C#][LeetCode] 8. String to Integer (atoi)

外部輸入一個字串，若開頭為數字的話或 +- 符號的話則轉換為 int 的型態，且若超過 int 的最大或最小值則取極限值為準。

我的第一版答案如下：

public class Solution {
    public int MyAtoi(string str) {
        var arrNum = "0123456789".ToList();
        var arrSymbol = "-".ToList();

        if (string.IsNullOrWhiteSpace(str))
        {
            return 0;
        }
        else
        {
            str = str.Trim();
        }

        bool isFirstAddSymbol = (str[0] == '+');

        if (isFirstAddSymbol)
        {
            str = str.Substring(1);
        }

        if (string.IsNullOrWhiteSpace(str))
        {
            return 0;
        }

        bool isFirstNum = (arrNum.IndexOf(str[0]) != -1);
        bool isFirstSymbol = (arrSymbol.IndexOf(str[0]) != -1);

        if ((isFirstAddSymbol && isFirstSymbol) ||
            (!isFirstNum && !isFirstSymbol) ||
            (isFirstSymbol && str.All(x => arrNum.IndexOf(x) == -1)) ||
            (isFirstSymbol && str.Length > 1 && (arrNum.IndexOf(str[1]) == -1)))
        {
            return 0;
        }

        string numString = string.Empty;
        str = isFirstSymbol ? str.Substring(1) : str;

        for (int i = 0; i < str.Length; i++)
        {

            bool isNum = (arrNum.IndexOf(str[i]) != -1);

            if (!isNum)
            {
                break;
            }

            if (numString == "0")
            {
                numString = str[i].ToString();
            }
            else
            {
                numString += str[i].ToString();
            }

        }

        int maxValueLength = int.MaxValue.ToString().Length;
        long ans = int.MaxValue;

        bool isMoreMax = (numString.Length > maxValueLength) || (long.Parse(numString) > int.MaxValue);

        if (!isMoreMax)
        {
            ans = long.Parse(numString);
        }

        if (isFirstSymbol && isMoreMax)
        {
            ans += 1;
        }

        return Convert.ToInt32(isFirstSymbol ? -ans : ans);
    }
}

參考最佳解答後的優化版：

public class Solution {
    public int MyAtoi(string str) {
        str = (str ?? string.Empty).Trim();
        
        if (string.IsNullOrWhiteSpace(str))
        {
            return 0;
        }
        
        bool isMinusSign = (str[0] == '-');
        if (isMinusSign || (str[0] == '+'))
        {
            
            if ((str.Length == 1) ||
                ((str.Length > 1) && (str[1] < '0' || str[1] > '9')))
            {
                return 0;
            }
            
            str = str.Substring(1);
        }
        
        string numString = string.Empty;
        foreach (var item in str)
        {
            bool isNum = (item >= '0' && item <= '9');
            
            if (!isNum) {
                break;
            }
            
            if (numString == "0")
            {
                numString = item.ToString();
            }
            else
            {
                numString += item.ToString();
            }
        }
        
        if (numString == string.Empty)
        {
            return 0;
        }

        int maxValueLength = int.MaxValue.ToString().Length;
        long ans = int.MaxValue;

        bool isMoreMax = (numString.Length > maxValueLength) || (long.Parse(numString) > int.MaxValue);

        if (!isMoreMax)
        {
            ans = long.Parse(numString);
        }

        if (isMinusSign && isMoreMax)
        {
            ans += 1;
        }

        return Convert.ToInt32(isMinusSign ? -ans : ans);
    }
}