C++版 - Leetcode 8: String to Integer (myAtoi，C库函数atoi模拟) (剑指offer 面试题49) 解题报告

#include<iostream>
#include<string>
using namespace std;

class Solution {
public:
    int myAtoi(string str)
	{
        if(str=="") return 0;  // str.length()==0 等价于 str==""，故条件不必写为if(str=="" || str.length()==0) 
        long long final, res=0;
        int len=str.length();
        int sign=1;
        int sign_count=0;
		for(int i=0; i<len; i++)
        {
        	char ch=str[i];
			if(ch=='-' || ch=='+') sign_count++;
			if(sign_count<=1)			
			{
			if(ch<='9' && ch>='0')
			{
				res = res*10+ch-'0';
	if(res> INT_MAX && sign==1) return INT_MAX;   // 当输入数超过INT_MAX或INT_MIN时，需及时退出，如果等计算完再返回，final会变为1(存储越界)
	if(res< INT_MIN && sign==-1) return INT_MIN;
				if(i+1<len && str[i+1]==' ') break;
			}
			else if(ch=='-' || ch=='+') 
			{
				if(ch=='-') sign=-1;
				if(i+1<len && str[i+1]==' ') break;
			}
				else if(ch==' ') ;  // 开头的空格忽略掉
					else break;  // 输入不合法，退出循环				
			}  	
			else return 0;
		}		
        if(res>= INT_MIN && res <= INT_MAX) final=res*sign;
        else if(sign==1) final = INT_MAX;
        	else if(sign==-1) final = INT_MIN;  // 如果越出整数边界，返回相应的值 
		return final; // -2147483648[(signed int)0x80000000] ~ 2147483647(0x7FFFFFFF)
    }   
};

// 以下为测试
int main()
{
	string str0; //	str0.size() == 0; // 后面的半句：可写可不写，字符串默认初始化为空串 
	string str1="1ab3";  // 应返回1
	string str2="18446744073709551617";	
	string str3="- +899999 ";
	string str4="    010"; // 应返回10
	string str5="   +0 123";
	string str6="13  456"; // 123
	string str7="   - 321";

	Solution sol;
	cout<<sol.myAtoi(str0)<<endl;	
	cout<<sol.myAtoi(str1)<<endl;
	cout<<sol.myAtoi(str2)<<endl;		
	cout<<sol.myAtoi(str3)<<endl;	
	cout<<sol.myAtoi(str4)<<endl;
	cout<<sol.myAtoi(str5)<<endl;
	cout<<sol.myAtoi(str6)<<endl;	
	cout<<sol.myAtoi(str7)<<endl;	
	return 0;
} 

class Solution {
public:
    int StrToInt(string str)
    {
        if(str=="") return 0;
        long long res=0;
        int len=str.length();
        int isPositive=1;
         
        for(int i=0; i<len; i++)
        {
            char ch=str[i];
            if(ch<='9' && ch>='0')  res = res*10+ch-'0';
            else if(ch=='-') isPositive=-1;
                else if(ch=='+') ;  // 不处理，继续往后扫
                    else return 0;
        }
        return res*isPositive;
    }
};

class Solution {
public:
    enum Status{kValid = 0,kInvalid};
    int g_nStatus = kValid;
    int StrToInt(string str) {
        g_nStatus = kInvalid;
        long long num = 0;
        int index=0;
        if((!str.empty())&&str[index]!='\0')
        {
            bool minus = false;
            if(str[index]=='+')
                index++;
            else if(str[index]=='-')
            {
                index++;
                minus = true;
            }
            if(str[index]!='\0')
            {
                int length=str.size();
                num = StrToIntCore(str.substr(index,length-index),minus);  
            }
        }
        return (int)num;
    }
         
    long long StrToIntCore(const string& digit,bool minus)
    {
        long long num=0;
        int i=0;
        while(digit[i]!='\0')
        {   //合法情况
            //if(digit[i]=='+'||digit[i]=='-')
                //i++;
            if(digit[i]>='0'&&digit[i]<='9')
            {
                int flag = minus?-1:1;
                num = num*10+ flag*(digit[i]-'0');
                //越界情况
                if((!minus&&num>0x7FFFFFFF)||(minus&&num<(signed int)0x80000000))
                {
                    num = 0;
                    break;
                }
                i++; 
            }
            else//非法情况
            {
                num = 0;
                break;
            }
        }
        if(digit[i]=='\0')
            g_nStatus = kValid;
        return num;
    }
};