Subset Sum

The subset sum problem and its description

Let’s consider a more complicated problem called subset sum. Given a set $X$ of positive integers and target integer $T$, is there a subset of elements in $X$ that adds up to $T$? Notice that there can be more than one such subset. For example, if $X = {8,6,7,5,3,10,9}$ and $T = 15$, the answer is True, because the subsets ${8,7}$ and ${7,5,3}$ and ${6,9}$ and ${5,10}$ all sum to 15. On the other hand, if $X = {11,6,5,1,7,13,12}$ and $T = 15$, the answer is False.

There are two trivial cases. If the target value $T$ is zero, then we can immediately return True because the empty set is a subset of every set $X$, and the elements of the empty set add up to zero. On the other hand, if $T < 0$, or if $T\neq 0$ but the set $X$ is empty, then we can immediately return False.

For the general case, consider an arbitrary element $x ∈ X$. (We’ve already handled the case where $X$ is empty.) There is a subset of $X$ that sums to $T$ if, and only if, one of the following statements is true:

• There is a subset of $X$ that includes $x$ and whose sum is $T$.
• There is a subset of $X$ that excludes $x$ and whose sum is $T$.

In the first case, there must be a subset of $X\space \backslash \left\{ x\right\}$ that sums to $T − x$; in the second case, there must be a subset of $X\space \backslash \left\{ x\right\}$ that sums to $T$. So, we can solve $SubsetSum(X,T)$ by reducing it to two simpler instances: $SubsetSum(X\space \backslash \left\{ x\right\}, T-x)$ and $SubsetSum(X\space \backslash \left\{ x\right\}, T)$. The resulting recursive algorithm is shown below.