1.将字符串转换成整数

           没什么难点，就是考虑的情况比较多。

#include<iostream> 
#include<cctype> 
#include<exception> 
#include<climits> 
#include<cstdio> 
using namespace std; 
 
long Str2Num(const char *str) 
{ 
     if(!str)throw new std::exception("NULL String"); 
     bool isNegetive = false; 
     int radix = 10; 
     unsigned long ret = 0; 
     if(*str == '-'){ 
         isNegetive = true; 
         ++str; 
     }else if(*str == '+')++str; 
     if(*str == '0'&&(str+1)&&*(str+1)!='x'&&*(str+1)!='0'){ 
         radix = 8; 
         ++str; 
     } 
     else if(*str == '0'&& tolower(*(str+1))=='x'){ 
         radix = 16; 
         ++str;++str; 
     } 
     for(;*str!='\0';++str){ 
         int ch = *str; 
         if((ch - '0') <radix){ 
              ret = ret*radix + ch - '0'; 
         } 
         else if(radix == 16 && ('a'<=tolower(ch)&&tolower(ch)<='f')){ 
              ret = ret*radix + tolower(ch) - 'a' + 10; 
         } 
         else 
              throw new exception("Invalid num"); 
         if(ret>LONG_MAX)throw new exception("overfolw"); 
     } 
     return isNegetive?(-ret):ret; 
} 

#include<iostream>
#include<cctype>
#include<exception>
#include<climits>
#include<cstdio>
using namespace std;

long Str2Num(const char *str)
{
     if(!str)throw new std::exception("NULL String");
     bool isNegetive = false;
     int radix = 10;
     unsigned long ret = 0;
     if(*str == '-'){
      isNegetive = true;
      ++str;
     }else if(*str == '+')++str;
     if(*str == '0'&&(str+1)&&*(str+1)!='x'&&*(str+1)!='0'){
      radix = 8;
      ++str;
     }
     else if(*str == '0'&& tolower(*(str+1))=='x'){
      radix = 16;
      ++str;++str;
     }
     for(;*str!='\0';++str){
      int ch = *str;
      if((ch - '0') <radix){
           ret = ret*radix + ch - '0';
         }
         else if(radix == 16 && ('a'<=tolower(ch)&&tolower(ch)<='f')){
           ret = ret*radix + tolower(ch) - 'a' + 10;
         }
         else
           throw new exception("Invalid num");
         if(ret>LONG_MAX)throw new exception("overfolw");
     }
     return isNegetive?(-ret):ret;
 

char * Num2Str(int Number) 
 
{ 
 
   char ch,*str,*right,*left; 
 
   unsigned int Value; 
 
   str = (char *)malloc(12*sizeof(char)); 
 
   left = right = str; 
 
   //如果是负数，则应加上负号，left、right向后走。 
 
   if(Number < 0) 
 
   { 
 
      Value = -Number; 
 
      *str = '-'; 
 
      left++,right++; 
 
   } 
 
   else 
 
      Value = (unsigned)Number; 
 
   //把数字转换成字符串（倒置的） 
 
   while(Value) 
 
   { 
 
      *right = (Value%10)+0x30; 
 
      Value = Value/10; 
 
      right++; 
 
   } 
 
   *right-- = '\0'; 
 
   //把倒置的字符串正放过来 
 
   while(right > left) 
 
   { 
 
      ch = *left; 
 
       *left++ = *right; 
 
       *right-- = ch; 
 
   } 
 
   return str; 
 
}