Prove that I can choose exactly one $\mathbf{1}$ from each row such that for every two of them,they are not in the same column.
There are some $1$ in the$ n\times n$ grid such that the number of $1$ in each row and each column is $K (K
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combinatorics
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2See [Hall’s marriage theorem](http://en.wikipedia.org/wiki/Hall%27s_marriage_theorem). – 2012-09-26
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0Please edit the question so the question is in the body, not just in the title. – 2012-09-27