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Questions related to combinatorics and discrete mathematical structures

2 votes
Accepted

Conditions for a binary tree being balanced

This is an interesting question. How do we define a family of binary trees is balanced? A generalized definition of balanced binary trees One of the most general definitions is that the height of a …
John L.'s user avatar
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1 vote
Accepted

Produce all unordered unique combinations of N-sized subsets of an m·N-sized set

Here is a program in Java that prints all unordered unique m-combinations of N-sized subsets of numbers from 0 inclusive to m * N exclusive. Click the run button to see some result. The algorithm use …
John L.'s user avatar
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6 votes
Accepted

Filling bins with pairs of balls

TL;DR -- No, there is no better strategy than the simple strategy. Here is the main idea of the proof. When there are not enough balls, there will be a "ball path" from a $k$-full bin to a bin wit …
John L.'s user avatar
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1 vote

Can all teams in a tournament finish with the same points? If yes, how can there also be few...

A simple algorithm that let all teams finish with same points Let $n$ teams sit around a circle evenly. If $n$ is odd, let each team win over the next $(n-1)/2$ teams clockwise. Otherwise $n$ is even …
John L.'s user avatar
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0 votes
Accepted

Lexicographic permutation

First, there is one special case of $k$, the powers of 10, which are 1, 10, 100, etc. Their positions at a lexicographically ordered sequence of positive integers are fixed. 1 is always at the 1st po …
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4 votes

Efficient n-choose-k random sampling

Here is the simplest algorithm, which is efficient when $k$ is much smaller than $n$ relatively. Input: two positive integers $n$ and $k$ with $k\le n$ Output: a random permutation of $k$ integers …
John L.'s user avatar
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0 votes
Accepted

Hiring problem from CLRS

In fact, your result is correct and that reference solution is wrong! Here is where that reference solution goes wrong. Since we view the candidate ranking as reading an random permutation, this …
John L.'s user avatar
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3 votes
Accepted

Primary/Secondary On Call Rotations

Given $n$ persons, this question is about a sequence of unordered pair of persons, called a two-person schedule(TPS). A TPS is good if any of its initial contiguous subsequences satisfies the followin …
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3 votes

Hard connected instances for Weisfeiler-Lehman test of isomorphism

One epic answer by David, one simplest answer by Yuval. However, there is no graphs. I mean, actual visual graphs. Here is the famous beautiful negative instances to 1-dimensional Weisfeiler-Lehman …
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1 vote
Accepted

Recover boolean vector from dot products

Here is an variant of the idea 1 in the question, where a factor of 2 is used to replace the original factor of 9. Let $v=[2^0, 2^1, 2^2, 2^3, \cdots, 2^{62}, 2^{63}, 0, 0, 0, \cdots]^T$. Obtain $b^ …
John L.'s user avatar
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2 votes
Accepted

The smallest periods of the prefixes of the Fibonacci word

What we need to prove are the smallest period for $s_1$ is 1, which is trivially true. the smallest period for $s_i$ is $F_{k-1}$ for all $F_k-1\le i\le F_{k+1}-2$, where $k\ge3$. Let $k\ge3$. You …
John L.'s user avatar
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1 vote
Accepted

Why 2 different edge min-cuts in an undirected multigraph must be completely disjoint?

Why 2 different edge min-cuts in an undirected multigraph must be completely disjoint? There is no "why" here since 2 different edge minimum-cuts in an undirected multigraph can have one edge in …
John L.'s user avatar
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