There is an invalid case also in Knapsack

[abhijitgawai]

Many people ignore it. Please add this invalid case where input is worng (when length of weight and value is not equal)

Describe your change:

There is invalid case in knapsack. Many people ignore it. I have included that case in knapsack.py
Invalid case is a case where weight and value array is not of equal length

Therefore I added - if len(wights)!=len(values): return -1

I have returned -1 because otherwise user will not get to know that input was wrong.
If I would have returned 0, user would think, there is no space in bag to accept least weight.

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Documentation change?

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