# Find the Nth Pure number

Given an integer N, the task is to find the Nth pure number.

A pure number has to satisfy three conditions:
1) It has even number of digits.
2) All digits are either 4 or 5.
3) And the number is a palindrome.

The Pure number series is: 44, 55, 4444, 4554, 5445, 5555, 444444, 445544, 454454, 455554 and so on.

Examples:

```Input: 5
Output: 5445
Explanation:
5445 is the 5th pure number in the series.

Input: 19
Output: 45444454
Explanation:
45444454 is the 19th pure number in the series.
```

## Recommended: Please try your approach on {IDE} first, before moving on to the solution.

Approach: We will assume that 2 numbers make one single block. For each block, there is 2block number of pure numbers. For pure numbers with 1 block, there are 21 pure numbers, for numbers with 2 blocks, there are 22 numbers and so on.

• Pure numbers starting with 4, starts at position 2block – 1 for example, 4444 is at (22 -1 = 3) which means it is at third position in the series.
• Pure numbers starting with 5 starts at position 2block + 2(block-1) -1 for example, 5555 is at (2^2 + 2^1 -1 =5) which means it is at fifth position in the series.

A pure number at a block is essentially sandwiched between two 4’s or 5’s and is a combination of all previous block numbers. To understand it better let’s consider the example below:

• First pure number is 44 and second pure number is 55
• 4444 (“4″+ “44” + “4”) 44 from previous block
• 4554 (“4″+ “55” + “4”) 55 from previous block
• 5445 (“5″+ “44” + “5”) 44 from previous block
• 5555 (“5″+ “55” + “5”) 55 from previous block

This pattern repeats for all the numbers in the series.

Below is the implementation of the above approach:

 `#include ` `using` `namespace` `std; ` ` `  `// CPP program to find ` `// the Nth pure num ` ` `  `// Function to check if it ` `// is a power of 2 or not ` `bool` `isPowerOfTwo(``int` `N) ` `{ ` `    ``double` `number = ``log``(N)/``log``(2); ` `    ``int` `checker = ``int``(number); ` `    ``return` `number - checker == 0; ` `} ` ` `  `// if a number belongs to 4 series ` `// it should lie between 2^blocks -1 to ` `// 2^blocks + 2^(blocks-1) -1 ` `bool` `isSeriesFour(``int` `N, ``int` `digits) ` `{ ` `    ``int` `upperBound = ``int``(``pow``(2, digits)+``pow``(2, digits - 1)-1); ` `    ``int` `lowerBound = ``int``(``pow``(2, digits)-1); ` `    ``return` `(N >= lowerBound) && (N < upperBound); ` `} ` ` `  `// Method to find pure number ` `string getPureNumber(``int` `N) ` `{ ` `    ``string numbers[N + 1]; ` ` `  `    ``numbers[0] = ``""``; ` ` `  `    ``int` `blocks = 0; ` `    ``int` `displacement = 0; ` ` `  `    ``// Iterate from 1 to N ` `    ``for` `(``int` `i = 1; i < N + 1; i++) { ` ` `  `        ``// Check if number is power of two ` `        ``if` `(isPowerOfTwo(i + 1)) { ` `            ``blocks = blocks + 1; ` `        ``} ` ` `  `        ``if` `(isSeriesFour(i, blocks)) { ` `            ``displacement ` `                ``= ``int``(``pow``(2, blocks - 1)); ` ` `  `            ``// Distance to previous ` `            ``// block numbers ` `            ``numbers[i] = ``"4"` `+ numbers[i - displacement] + ``"4"``; ` `        ``} ` ` `  `        ``else` `{ ` ` `  `            ``displacement = ``int``(``pow``(2, blocks)); ` ` `  `            ``// Distance to previous ` `            ``// block numbers ` `            ``numbers[i] = ``"5"` `+ numbers[i - displacement] + ``"5"``; ` `        ``} ` `    ``} ` ` `  `    ``return` `numbers[N]; ` `} ` ` `  `// Driver Code ` `int` `main() ` `{ ` `    ``int` `N = 5; ` ` `  `    ``string pure = getPureNumber(N); ` ` `  `    ``cout << pure << endl; ` `} ` ` `  `// This code is contributed by Surendra_Gangwar `

 `// Java program to find ` `// the Nth pure number ` ` `  `import` `java.io.*; ` ` `  `class` `PureNumbers { ` ` `  `    ``// Function to check if it ` `    ``// is a power of 2 or not ` `    ``public` `boolean` `isPowerOfTwo(``int` `N) ` `    ``{ ` `        ``double` `number ` `            ``= Math.log(N) / Math.log(``2``); ` `        ``int` `checker = (``int``)number; ` `        ``return` `number - checker == ``0``; ` `    ``} ` ` `  `    ``// if a number belongs to 4 series ` `    ``// it should lie between 2^blocks -1 to ` `    ``// 2^blocks + 2^(blocks-1) -1 ` `    ``public` `boolean` `isSeriesFour( ` `        ``int` `N, ``int` `digits) ` `    ``{ ` `        ``int` `upperBound ` `            ``= (``int``)(Math.pow(``2``, digits) ` `                    ``+ Math.pow(``2``, digits - ``1``) ` `                    ``- ``1``); ` `        ``int` `lowerBound ` `            ``= (``int``)(Math.pow(``2``, digits) ` `                    ``- ``1``); ` `        ``return` `(N >= lowerBound) ` `            ``&& (N < upperBound); ` `    ``} ` ` `  `    ``// Method to find pure number ` `    ``public` `String getPureNumber(``int` `N) ` `    ``{ ` `        ``String[] numbers ` `            ``= ``new` `String[N + ``1``]; ` ` `  `        ``numbers[``0``] = ``""``; ` ` `  `        ``int` `blocks = ``0``; ` `        ``int` `displacement = ``0``; ` ` `  `        ``// Iterate from 1 to N ` `        ``for` `(``int` `i = ``1``; i < N + ``1``; i++) { ` ` `  `            ``// Check if number is power of two ` `            ``if` `(isPowerOfTwo(i + ``1``)) { ` `                ``blocks = blocks + ``1``; ` `            ``} ` ` `  `            ``if` `(isSeriesFour(i, blocks)) { ` `                ``displacement ` `                    ``= (``int``)Math.pow( ` `                        ``2``, blocks - ``1``); ` ` `  `                ``// Distance to previous ` `                ``// block numbers ` `                ``numbers[i] ` `                    ``= ``"4"` `                      ``+ numbers[i - displacement] ` `                      ``+ ``"4"``; ` `            ``} ` `            ``else` `{ ` ` `  `                ``displacement ` `                    ``= (``int``)Math.pow( ` `                        ``2``, blocks); ` ` `  `                ``// Distance to previous ` `                ``// block numbers ` `                ``numbers[i] ` `                    ``= ``"5"` `                      ``+ numbers[i - displacement] ` `                      ``+ ``"5"``; ` `            ``} ` `        ``} ` ` `  `        ``return` `numbers[N]; ` `    ``} ` ` `  `    ``// Driver Code ` `    ``public` `static` `void` `main(String[] args) ` `        ``throws` `Exception ` `    ``{ ` `        ``int` `N = ``5``; ` ` `  `        ``// Create an object of the class ` `        ``PureNumbers ob = ``new` `PureNumbers(); ` ` `  `        ``// Function call to find the ` `        ``// Nth pure number ` `        ``String pure = ob.getPureNumber(N); ` ` `  `        ``System.out.println(pure); ` `    ``} ` `} `

 `// C# program to find ` `// the Nth pure number ` `using` `System; ` `  `  `class` `PureNumbers { ` `  `  `    ``// Function to check if it ` `    ``// is a power of 2 or not ` `    ``public` `bool` `isPowerOfTwo(``int` `N) ` `    ``{ ` `        ``double` `number ` `            ``= Math.Log(N) / Math.Log(2); ` `        ``int` `checker = (``int``)number; ` `        ``return` `number - checker == 0; ` `    ``} ` `  `  `    ``// if a number belongs to 4 series ` `    ``// it should lie between 2^blocks -1 to ` `    ``// 2^blocks + 2^(blocks-1) -1 ` `    ``public` `bool` `isSeriesFour( ` `        ``int` `N, ``int` `digits) ` `    ``{ ` `        ``int` `upperBound ` `            ``= (``int``)(Math.Pow(2, digits) ` `                    ``+ Math.Pow(2, digits - 1) ` `                    ``- 1); ` `        ``int` `lowerBound ` `            ``= (``int``)(Math.Pow(2, digits) ` `                    ``- 1); ` `        ``return` `(N >= lowerBound) ` `            ``&& (N < upperBound); ` `    ``} ` `  `  `    ``// Method to find pure number ` `    ``public` `string` `getPureNumber(``int` `N) ` `    ``{ ` `        ``string``[] numbers ` `            ``= ``new` `string``[N + 1]; ` `  `  `        ``numbers[0] = ``""``; ` `  `  `        ``int` `blocks = 0; ` `        ``int` `displacement = 0; ` `  `  `        ``// Iterate from 1 to N ` `        ``for` `(``int` `i = 1; i < N + 1; i++) { ` `  `  `            ``// Check if number is power of two ` `            ``if` `(isPowerOfTwo(i + 1)) { ` `                ``blocks = blocks + 1; ` `            ``} ` `  `  `            ``if` `(isSeriesFour(i, blocks)) { ` `                ``displacement ` `                    ``= (``int``)Math.Pow( ` `                        ``2, blocks - 1); ` `  `  `                ``// Distance to previous ` `                ``// block numbers ` `                ``numbers[i] ` `                    ``= ``"4"` `                      ``+ numbers[i - displacement] ` `                      ``+ ``"4"``; ` `            ``} ` `            ``else` `{ ` `  `  `                ``displacement ` `                    ``= (``int``)Math.Pow( ` `                        ``2, blocks); ` `  `  `                ``// Distance to previous ` `                ``// block numbers ` `                ``numbers[i] ` `                    ``= ``"5"` `                      ``+ numbers[i - displacement] ` `                      ``+ ``"5"``; ` `            ``} ` `        ``} ` `  `  `        ``return` `numbers[N]; ` `    ``} ` `  `  `    ``// Driver Code ` `    ``public` `static` `void` `Main() ` `    ``{ ` `        ``int` `N = 5; ` `  `  `        ``// Create an object of the class ` `        ``PureNumbers ob = ``new` `PureNumbers(); ` `  `  `        ``// Function call to find the ` `        ``// Nth pure number ` `        ``string` `pure = ob.getPureNumber(N); ` `  `  `        ``Console.Write(pure); ` `    ``} ` `} ` ` `  `// This code is contributed by chitranayal `

Output:
```5445
```

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Improved By : chitranayal, SURENDRA_GANGWAR
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