## Partition problem is to find whether the given set can be divided into two sets whose sum of elements in the subsets is equal.

### Example

**a)** Input array: {4, 5, 11, 9, 8, 3}

Output: Yes

The array can be divided into 2 subsets with equal sum {4, 5, 11} and {9, 8, 3}

**b)** Input array: {2, 4, 11, 9, 8, 3}

Output: No

The array cannot be divided into 2 subsets with equal sum.

## Method 1

**Time complexity: O(2*n)**

## Algorithm

**a.** Create ispartition function to check whether it contains 2 subsets with equal sum or not.

**b.** In this function,

1) Calculate the sum of elements in the array.

2) If sum is odd then return false.

3) Else call SubsetSum on the array with sum = sum/2.

**c.** SubsetSum is to find whether there is a subset in the array with sum equal to given Sum.

**d.** In this function SubsetSum use recursive approach,

1) If last element is greater than the sum, then ignore it and move on by reducing size to size -1.

2) Else, check sum can be obtained including the last element or excluding the last element.

3) If sum = 0, return true.

4) If size = 0 but sum not equal to zero return false.

## C++ Program

```
#include <bits/stdc++.h>
using namespace std;
//recursive method
bool SubsetSum(int array[], int n, int sum)
{
if (sum == 0)
{
return true;
}
if(n == 0 && sum != 0)
{
return false;
}
//last element greater than the sum remove it and continue
if (array[n-1] > sum)
{
return SubsetSum(array, n-1, sum);
}
//include last or exclude last
return SubsetSum(array, n-1, sum) || SubsetSum (array, n-1, sum-array[n-1]);
}
//if sum of elements is odd return false
//else find if there is a subset with sum = sum/2
bool isPartiion(int array[], int n)
{
int sum = 0;
for(int i = 0; i < n; i++)
{
sum += array[i];
}
if(sum%2 != 0)
{
return false;
}
return SubsetSum(array, n, sum/2);
}
//Main function
int main()
{
int input_array[] = {2, 3, 8, 7};
int size = sizeof(input_array)/sizeof(input_array[0]);
bool x = isPartiion(input_array,size);
if(x)
{
cout<<"given input array can be divided into two subsets with equal sum";
}
else
{
cout<<"given input array cannot be divided into two subsets with equal sum";
}
return 0;
}
```

## Method 2

**Time complexity: O(sum*n)**

## Algorithm

Here we use dynamic programming,

**a)** Create a 2D array partition_array size sum/2 + 1 and n+1.

**b)** In this array,

1) Store true if a subset of elements till array[j-1] has sum equal to i.

2) Else, store false.

**c)** Return partition_array[sum/2][n].

### Algorithm working

## C++ Program

```
#include <bits/stdc++.h>
using namespace std;
//dynamic programming
bool isPartiion(int array[], int n)
{
int sum = 0,i, j;
//sum = sum of elements in the array
for (i = 0; i < n; i++)
{
sum += array[i];
}
//if sum is odd return false
if (sum%2 != 0)
{
return false;
}
//partition table 2d array
bool partition_array[sum/2+1][n+1];
for (i = 0; i <= n; i++)
{
partition_array[0][i] = true;
}
for (i = 1; i <= sum/2; i++)
{
partition_array[i][0] = false;
}
for (i = 1; i <= sum/2; i++)
{
for (j = 1; j <= n; j++)
{
partition_array[i][j] = partition_array[i][j-1];
if(i >= array[j-1])
{
partition_array[i][j] = partition_array[i][j] || partition_array[i - array[j-1]][j-1];
}
}
}
// uncomment this part to print table
for (i = 0; i <= sum/2; i++)
{
for (j = 0; j <= n; j++)
printf ("%4d", partition_array[i][j]);
printf("\n");
}
return partition_array[sum/2][n];
}
//Main
int main()
{
int input_array[] = {1, 2, 3, 6};
int size = sizeof(input_array)/sizeof(input_array[0]);
bool x = isPartiion(input_array,size);
if(x)
{
cout<<"given input array can be divided into two subsets with equal sum";
}
else
{
cout<<"given input array cannot be divided into two subsets with equal sum";
}
}
```