Leetcode——8. 字符串轉換整數 (atoi)

題目傳送門
思路一：就簡單的面向過程但是要處理好邊界問題比較費勁

class Solution
{
public:
    int myAtoi(string str)
    {
        int strLength=str.length();
        int i;
        for(i=0;i<strLength;i++)
        {
            if(str[i]==' ')
            {
                continue;
            }
            else
            {
                break;
            }
        }
        if(str[i]=='+')
        {
            i++;
            int temp=i;
            while(int(str[i])>47&&int(str[i])<58)
            {
                i++;
            }
            string tempstr=str.substr(temp,i-temp);
            int j=0;
            while(tempstr[j]=='0')
            {
                j++;
            }
            tempstr=tempstr.substr(j);
            if(tempstr.length()>10)
            {
                return INT_MAX;
            }
            if(tempstr.length()==0)
            {
                return 0;
            }
            if(stol(tempstr)>INT_MAX)
            {
                return INT_MAX;
            }
            else
            {
                return stol(tempstr);
            }
        }
        else if(str[i]=='-')
        {
            i++;
            int temp=i;
            while(int(str[i])>47&&int(str[i])<58)
            {
                i++;
            }
            string tempstr=str.substr(temp,i-temp);
            int j=0;
            while(tempstr[j]=='0')
            {
                j++;
            }
            tempstr=tempstr.substr(j);
            if(tempstr.length()>10)
            {
                return INT_MIN;
            }
            if(tempstr.length()==0)
            {
                return 0;
            }
            if(-1*stol(tempstr)<INT_MIN)
            {
                return INT_MIN;
            }
            else
            {
                return -1*stol(tempstr);
            }
        }
        else if(int(str[i])>47&&int(str[i])<58)
        {
            int temp=i;
            while(int(str[i])>47&&int(str[i])<58)
            {
                i++;
            }
            string tempstr=str.substr(temp,i-temp);
            int j=0;
            while(tempstr[j]=='0')
            {
                j++;
            }
            tempstr=tempstr.substr(j);
            if(tempstr.length()==0)
            {
                return 0;
            }
            if(tempstr.length()>10)
            {
                return INT_MAX;
            }            
            if(stol(tempstr)>INT_MAX)
            {
                return INT_MAX;
            }
            return stol(tempstr);
        }
        else
        {
            return 0;
        }
    }
};

思路二：有窮自動機

class Automation
{
    public:
        long ans=0;
        int sign=1;
        void getc(char ch)
        {
            int temp=whatChar(ch);
            string curstate=m[state][temp];
            state=curstate;
            if(curstate=="sign")
            {
                if(ch=='-')
                {
                    sign=-1;
                }
            }
            if(curstate=="number")
            {
                ans=ans*10+ch-'0';
                if(ans>INT_MAX&&sign==1)
                {
                    ans=INT_MAX;
                }
                if(-1*ans<(INT_MIN)&&sign==-1)
                {
                    ans=-1*(long)INT_MIN;
                }
            }
        }
    private:
        string state="start";
        unordered_map<string,vector<string> > m=
        {
            {"start",{"start","sign","number","end"}},
            {"sign",{"end","end","number","end"}},
            {"number",{"end","end","number","end"}},
            {"end",{"end","end","end","end"}}
        };
        int whatChar(char ch)
        {
            if(ch==' ')
            {
                return 0;
            }
            else if(ch=='+'||ch=='-')
            {
                return 1;
            }
            else if(int(ch)>47&&int(ch)<58)
            {
                return 2;
            }
            else
            {
                return 3;
            } 
        }
};

class Solution
{
public:
    int myAtoi(string str)
    {
        Automation automation;
        for(int i=0;i<str.length();i++)
        {
            automation.getc(str[i]);
        }
        return automation.sign*automation.ans;
    }
};