请实现一个函数，将一个字符串中的空格替换成“%20”。例如，当字符串为We Are Happy.则经过替换之后的字符串为We%20Are%20Happy

问题：请实现一个函数，将一个字符串中的空格替换成“%20”。例如，当字符串为We Are Happy.则经过替换之后的字符串为We%20Are%20Happy

用C/C++编写的程序实现

class Solution {
public:
 void replaceSpace(char *str,int length) {

     if (str==NULL||length<0)
     {
         return;
     }
          int i=0;
          int oldnumber=0;
          int blackkey=0;
        while(str[i]!='\0')
            {
          oldnumber++;
          if (str[i]==' ')
             blackkey++;
            i++;
        }
        int newnumber=oldnumber+blackkey*2;
         if(newnumber>length)//长度大于总长度就无法插入
            return ;
       
       
        while(oldnumber>=0&&newnumber>oldnumber)
        {
            if (str[oldnumber]==' ')
                {
                    str[newnumber--]='0';
                    str[newnumber--]='2';
                    str[newnumber--]='%';
                }
            else
                str[newnumber--]=str[oldnumber];
               oldnumber--;
           
        }
    }
};