Question: Given a **non-empty** array of integers `nums`

, every element appears *twice* except for one. Find that single one.

## Solution

## Approach 1: Hash Table

We use hash table to avoid the O(n)* *time required for searching the elements.

Iterate through all elements in

`nums`

and set up key/value pair.Return the element which appeared only once.

## Approach 2: Math

We can use the following equation to solve the problem:

2 ∗ ( *a *+ *b *+ *c *) − ( *a *+ *a *+ *b *+ *b *+ *c *) = *c *