C语言字符串逆序输出问题

#include"stdio.h"
#include"string.h"
#include <conio.h>
int main(){
char a[100];
int i, j, prev = 0, back = -2, length = 0, tmp;
printf("Please input the char string.\n");
gets(a);
for(length = 0; a[length] != '\0'; length ++);
/* for(i = 0; i < length/2; i ++){
tmp = a[length - i - 1];
a[length - i - 1] = a[i];
a[i] = tmp;
}*/
for(i = 0; i < length; i ++){
if(a[i] == ' '){
prev = back + 2;
back = i - 1;
for(j = 0; j <= ((back - prev)/2); j ++){
tmp = a[prev + j];
a[prev + j] = a[back - j];
a[back - j] = tmp;
}
}
}
for(j = 0; j <= (length - 1 - back - 2)/2; j ++){
tmp = a[back + 2 + j];
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