LuckyXor | Topcoder Archive

Problem Statement

A lucky number is a positive integer consisting of only the digits 4 and 7.

Given an int a, return an int b strictly greater than a, such that a XOR b is a lucky number. (See Notes for the definition of XOR.) The number b should be in the range 1 to 100, inclusive. If such a number does not exist, return -1. If there are multiple such b, you may return any of them.

Definition

Class:: LuckyXor
Method:: construct
Parameters:: int
Returns:: int
Method signature:: int construct(int a)
(be sure your method is public)

Notes

XOR is the bitwise exclusive-or operation. To compute the value of P XOR Q, we first write P and Q in binary. Then, each bit of the result is computed by applying XOR to the corresponding bits of the two numbers, using the rules 0 XOR 0 = 0, 0 XOR 1 = 1, 1 XOR 0 = 1, and 1 XOR 1 = 0.
For example, let's compute 21 XOR 6. In binary these two numbers are 10101 and 00110, hence their XOR is 10011 in binary, which is 19 in decimal.
You can read more about the XOR operation here: https://en.wikipedia.org/wiki/Exclusive_or

Constraints

a is between 1 and 100, inclusive.

Examples

```
4
```
```
Returns: 40
```
4 XOR 40 = 44, 44 is a lucky number.
```
19
```
```
Returns: 20
```
19 XOR 20 = 7
```
88
```
```
Returns: 92
```
88 XOR 92 = 4
```
36
```
```
Returns: -1
```

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