1 /* 2 atoi算法，要求完美版 3 有两种，一种是用longlong，一种是真用int 4 “” 5 " " 6 “-” 7 “+” 8 “ -23” 9 “ +23” 10 “12a" 11 "abc" 12 越界情况 13 */ 14 #include <iostream> 15 using namespace std; 16 17 //方法一用 long long 取巧 18 int atoi(const char * str,bool & flag) 19 { 20 long long result=0; 21 int tmp; 22 int sign=1; 23 if(str==NULL) 24 { 25 flag=false; 26 return 0; 27 } 28 while(isspace(*str)) 29 str++; 30 //if(*str==' ') 31 //{ 32 // flag=false; 33 // return 0; 34 //} 35 if(*str=='+'||*str=='-') 36 { 37 if(*str=='-') 38 sign=-1; 39 str++; 40 } 41 if(*str==' ') //” “和”+“ ，”-“的情况 42 { 43 flag=false; 44 return 0; 45 } 46 while(*str>='0'&&*str<='9') 47 { 48 tmp=*str-'0'; 49 result=result*10+tmp; 50 if(sign==-1) 51 { 52 if(result-1>INT_MAX) //这里注意，如果是INT_MAX+1，系统或默认转换成int，导致越界，所以用result-1，result是longlong不会越界 53 { 54 result=INT_MAX+1; 55 flag=false; 56 break; 57 } 58 } 59 else if(result>INT_MAX) 60 { 61 result=INT_MAX; 62 flag=false; 63 break; 64 } 65 str++; 66 } 67 if(*str==' ') 68 flag=true; 69 else 70 flag=false; 71 return sign*result; 72 } 73 74 //方法二 用int 75 int atoi2(const char * str ,bool & flag) 76 { 77 int result=0; 78 int sign=1; 79 int tmp; 80 int MAX_DIV=INT_MAX/10; 81 int MAX_R=INT_MAX%10; 82 int MIN_R=MAX_R+1; //或者为MIN_R=-(INT_MIN%10); //如果用(INT_MAX+1)%10就错了，直接加的时候越界 83 int MIN_DIV=(INT_MAX)/10; 84 85 //july用的是max=(int)((unsigned)~0>>1); 86 //min_r=(int)(((unsigned)~0>>1+1)%10); 87 if(str==NULL) 88 { 89 flag=false; 90 return result; 91 } 92 while(isspace(*str)) 93 str++; 94 if(*str=='+'||*str=='-') 95 { 96 if(*str=='-') 97 sign=-1; 98 str++; 99 } 100 if(*str==' ') 101 { 102 flag=false; 103 return result; 104 } 105 while(*str>='0'&&*str<='9') 106 { 107 int tmp=*str-'0'; 108 if(sign==1) 109 { 110 if(result>MAX_DIV||(result==MAX_DIV&&tmp>MAX_R)) 111 { 112 result=INT_MAX; 113 flag=false; 114 return result; 115 } 116 } 117 else if(result>MIN_DIV||(result==MIN_DIV&&tmp>MIN_R)) 118 { 119 result=INT_MIN; //result直接为最小-2147483648，直接返回了 120 flag=false; 121 return result; 122 } 123 result=result*10+tmp; 124 str++; 125 } 126 if(*str==' ') 127 flag=true; 128 else 129 flag=false; 130 return sign*result; 131 } 132 133 134 135 int main() 136 { 137 char * str[]={""," ","abc"," 123"," -1234"," +34"," 12bc","+","-","+++","0","2147483647","+2147483648","-2147483648","-2147483649","+12123131312432342"," -122323434534534654"}; 138 //cout<<INT_MAX<<endl; 139 int n=17; 140 bool flag; 141 for(int i=0;i<n;i++) 142 { 143 int tmp=atoi2(str[i],flag); 144 if(tmp==0) 145 { 146 if(flag==false) 147 cout<<str[i]<<" 错误数据转换为： "<<tmp<<endl; 148 else 149 cout<<str[i]<<" 转换为: "<<tmp<<endl; 150 } 151 else 152 { 153 cout<<str[i]<<" 转换为: "<<tmp<<endl; 154 } 155 } 156 system("pause"); 157 }