String to Integer (atoi)

Implement atoi to convert a string to an integer.

Hint: Carefully consider all possible input cases. If you want a challenge, please do not see below and ask yourself what are the possible input cases.

Notes: It is intended for this problem to be specified vaguely (ie, no given input specs). You are responsible to gather all the input requirements up front.

class Solution {
public:
    int atoi(string str) {
        long long result=0;
        int flag=1;
        int i=0;
        while(str[i]==' '&&str[i]!=''){
            i++;
        }
        if(str[i]=='-'&&str[i]!=''){
            flag=0;
            i++;
        }else if(str[i]=='+'&&str[i]!=''){
            i++;
        }
        for(;str[i]!='';i++){
            if(str[i]>='0'&&str[i]<='9'){
                
                
                if(flag){
                    result=result*10+str[i]-'0';
                    if(result>0x7FFFFFFF)
                    return 0x7FFFFFFF;
                }else{
                    result=result*10-(str[i]-'0');
                    if(result<(signed int)0x80000000)
                    return 0x80000000;
                } 
            }else break;
        }
        return (int)result;
        
        
    }
};

　注意：

1、字符前的空格处理；

2、无效字符输入

3、整数越界问题，此处使用了小技巧，将返回类型定义为long long。再分开判断是否越界

32位整数范围（（signed int） 0x80000000~0x7FFFFFFF）