Reversing a Sentence in C

#include<stdio.h>
#include<conio.h>
#include<string.h>
const int opt = 50;
int main(){
int i=0,j=0,c=0,vali[opt] = {0};
char str1[opt],str2[opt]="",str3[opt]="",ch;
printf("Enter the sentence:");
gets(str1);
for(i=0;str1[i] != '\0';i++){
if(str1[i] == ' '){
vali[j+1] = i;
j++;
}
}
c = j;
for(i=0;i<=j && c!=0;i++){
strcat(str2,&str1[vali[c]+1]);
strcpy(&str1[vali[c]],str3);
strcat(str2," ");
c--;
}
strcat(str2,str1);
printf("\nYour Reversed Sentence Is:");
puts(str2);
getch();
return 0;
}