#include <stdio.h>

 void ReplaceBlank(char string[],int length){

         if(string == NULL || length == 0){

                 return;

         }

         int originalLength = 0;//

         int blankCount = 0;

         while(string[originalLength] != '\0'){

                 if(string[originalLength] == ' '){

                         ++blankCount;

                 }

                 ++originalLength;

         }

         int newLength = originalLength + blankCount * 2;

         if(newLength > length){//not enough space

                 return;

         }

         int indexFirst = originalLength;

         int indexSecond = newLength;

         while(indexFirst>=0 && indexSecond>indexFirst){

                 if(string[indexFirst] == ' '){

                         string[indexSecond--] = '0';

                         string[indexSecond--] = '2';

                         string[indexSecond--] = '%';

                 }else{

                         string[indexSecond--] = string[indexFirst];

                 }

                 --indexFirst;

         }

 }

 int main(){

         char str[50] = "We are happy.";

         printf("str = %s\n",str);

         ReplaceBlank(str,50);

         printf("str = %s\n\n",str);

         char str1[50] = "Wearehappy.";

         printf("str1 = %s\n",str1);

         ReplaceBlank(str1,50);

         printf("str1 = %s\n\n",str1);

         char str2[50] = "   We are happy.";

         printf("str2 = %s\n",str2);

         ReplaceBlank(str2,50);

         printf("str2 = %s\n\n",str2);

         char str3[50] = "";

         printf("str3 = %s\n",str3);

         ReplaceBlank(str3,50);

         printf("str3 = %s\n\n",str3);

         return 0;

 }

str = We are happy.

str = We%20are%20happy.

str1 = Wearehappy.

str1 = Wearehappy.

str2 =    We are happy.

str2 = %%%20We%20are%20happy.

str3 =

str3 =