# Array of Doubled Pair Problem in Java

It is a problem frequently asked in interviews of top IT companies like Google, Amazon, TCS, Accenture, etc. By solving the problem, one wants to check the logical ability, critical thinking, and problem-solving skill of the interviewee. So, in this section, we are going to solve an array of doubled pair problem in Java with different approaches and logic. Also, we will create Java programs for the same.

## Problem Statement

In this problem, we have given an integer array (arr[]) of even length. In the array, we have to check if it is possible to reorder the array that must satisfy the following condition:

For every 0 < = I < len(arr)/2

If the above conditions satisfy, return true, else return false.

Example 1:

Input: a[] = [1, 2, 3, 4, 5, 6, 7]

Output: false

Example 2:

Input: a[] = [1, 4, 8, 12, 14, 18]

Output: false

Example 3:

Input: a[] = [4, -2, 2, -4]

Output: true

Explanation: We can take two groups, [-2, -4] and [2, 4] to form [-2, -4, 2, 4] or [2, 4, -2, -4].

Let's understand the problem in an easy way.

Suppose, x is an array element with the least absolute value, there must exist a pair with the element 2*x, as there does not exist any other element x/2 to pair with it.

Let's check elements in order of absolute value. When we check an element x and it isn't used, it must pair with 2*x. We will try to write x, 2x. If we fail to do so, then return false. If we write everything, then return true. In order to track what we have not written yet, use a count variable to store it.

Let's understand it through examples.

### Example

Case 1: If an array has positive elements

Consider an array [2, 4, 4, 8]. We have one x = 2, we need to match it with one 2x = 4. Then one 4 is gone, and we have the other x = 4. We need to match it with one 2x = 8. There are no other elements to match.

In this array, we have picked the first element of the array because it's the smallest element of the array and there is no x/2 left. So, we need to find 2x.

Case 2: If an array has negative elements

One way is that start from the biggest element (with absolute value smallest), and we apply the same logic (as above). Another way is, start from the smallest element (with absolute value biggest), and try to find x/2 each turn.

### Explanation

1. Count all numbers.
2. Loop all numbers on the order of its absolute. We have count[x]of x, so we need the same number of 2x to match them.
3. If count[x] > count[2 * x], then we return false.
4. If count[x] <= count[2 * x], then we do count[2 * x] -= count[x]to remove matched 2x.

Here, we need not to care about 0's. It does not fit in the above logic but it will not break the algorithm.

• If count is odd, it will not get matched in the end. (Anyway, we can return false earlier here)
• If count is even, we still have count <= count[2 * 0]. And we still need to check all other numbers.

## Algorithm

1. Sort the given array using a custom comparator.
2. Make a custom comparator function `compare` to compare two numbers based on their absolute value.
3. Initialize a map `frequency`.
4. Loop over the array 'ARR' and add one count to every occurrence of every array element.
5. Loop over the array again and add the following conditions:
• If the value associated with the 'ARR[i]' as a key is zero, then continue.
• If the value associated with the 2 * 'ARR[i]' as a key is zero, then return false.
• Else decrease the count by one for the 'ARR[i]' and 2 * 'ARR[i]' as a key in the Map.

## Java Program to Check Array of Doubled Pair

DoubledPair1.java

Output:

```false
```

Let's see another logic for the same.

DoubledPair2.java

Output:

```true
```

## Complexity Analysis

The runtime is O(N * logN), constructing the tree map is the dominant part of the runtime.

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