PickTwoCards | Topcoder Archive

Problem Statement

You are given a String cards, where each character represents the color of a card. You then select two of those cards at random. Return the probability that they are the same color.

Definition

Class:: PickTwoCards
Method:: equalProbability
Parameters:: String
Returns:: double
Method signature:: double equalProbability(String cards)
(be sure your method is public)

Notes

A return value that is less than 1e-9 absolute or relative error is considered correct.

Constraints

cards will contain between 2 and 50 characters, inclusive.
Each character of cards will be between 'A' and 'Z', inclusive.

Examples

```
"AAB"
```
```
Returns: 0.3333333333333333
```
There are three ways to pick two cards, one of which is both "A" cards.
```
"ABC"
```
```
Returns: 0.0
```
Can't get two the same when there's only one of each.
```
"AA"
```
```
Returns: 1.0
```
Only two cards to choose, so we always get a matched pair.
```
"ABABA"
```
```
Returns: 0.4
```

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