ZeroCombination need help

[desert564]

pls help me with this
the user enters a number n.consider the sequence from 1 through n in ascending order.insert either '+' or'-' between each of the digits so that the resultant sum is zero.
for eg:
input=7
output= 1+2-3+4-5-6+7=0
1+2-3-4+5+6-7=0
1-2+3+4-5+6-7=0
1-2-3-4-5+6+7=0
however when i programmed it i only got the first two outputs.the ones where + is the first operator..but the code didn't print the next two...pls help or find an error in the following code(it's happened with other inputs too)

import java.io.*;
public class zerocombination
{
public String toBin(int x)// this function converts a given number to its binary form
{
String str="";
while(x!=0)// dividing by base and building up the string in reverse eventually gives the binary form
{
str=(x%2)+str;
x/=2;
}
return str;
}
public void main()throws IOException
{
BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
System.out.println("ENTER A NUMBER");
int num=Integer.parseInt(br.readLine());// accepting a number
int n=(int)(Math.pow(2,num-1));
boolean flag=false;
String str[]=new String[n];// creating array in which combination s will be stored
String copy="";
// following loop converts all numbers from 0 to 2^(n-1) to binary.the binary forms represents all the combinations in which the consecutive numbers can be added 
for(int i=0;i<n;i++)
{
str[i]=toBin(i);
while(str[i].length()<num)
str[i]="0"+str[i];
int exp=1;
String ex="1";
for(int j=2;j<=num;j++)
{
int x=((int)(str[i].charAt(j-2)))-48;
if(x==0)
{
ex+="+"+j;
exp+=j;
}
else
{
exp-=j;
ex+="-"+j;
}
}
// this is done to prevent repetition
if(exp==0&&ex.equals(copy)==false)
{
copy=ex;
flag=true;
System.out.println(ex+"=0");
}
}
if(flag==false)
System.out.println("NO SUCH EXPRESSION");
System.out.println();System.out.println();
}
}

[NormR1]

Is Your problem:
Finding the correct algorithm?
Or is it writing a correct program to follow your algorithm?

Can you explain the algorithm you are trying to program? What are the steps you need to take to solve the problem?

[NormR1]

Can you edit your post and wrap the code in code tags to preserve the formatting.
The posted code is hard to read.