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2097. Valid Arrangement of Pairs

SourceWeekly Contest 270 Q4DifficultyHardRating2650

Description

You are given a 0-indexed 2D integer array pairs where pairs[i] = [starti, endi]. An arrangement of pairs is valid if for every index i where 1 <= i < pairs.length, we have endi-1 == starti.

Return any valid arrangement of pairs.

Note: The inputs will be generated such that there exists a valid arrangement of pairs.

 

Example 1:

Input: pairs = [[5,1],[4,5],[11,9],[9,4]]
Output: [[11,9],[9,4],[4,5],[5,1]]
Explanation:
This is a valid arrangement since endi-1 always equals starti.
end0 = 9 == 9 = start1 
end1 = 4 == 4 = start2
end2 = 5 == 5 = start3

Example 2:

Input: pairs = [[1,3],[3,2],[2,1]]
Output: [[1,3],[3,2],[2,1]]
Explanation:
This is a valid arrangement since endi-1 always equals starti.
end0 = 3 == 3 = start1
end1 = 2 == 2 = start2
The arrangements [[2,1],[1,3],[3,2]] and [[3,2],[2,1],[1,3]] are also valid.

Example 3:

Input: pairs = [[1,2],[1,3],[2,1]]
Output: [[1,2],[2,1],[1,3]]
Explanation:
This is a valid arrangement since endi-1 always equals starti.
end0 = 2 == 2 = start1
end1 = 1 == 1 = start2

 

Constraints:

  • 1 <= pairs.length <= 105
  • pairs[i].length == 2
  • 0 <= starti, endi <= 109
  • starti != endi
  • No two pairs are exactly the same.
  • There exists a valid arrangement of pairs.

Solutions

Solution 1

Thinking

Pairs must chain, i.e. an Eulerian path in the directed graph of edges \(start \to end\). Hierholzer runs in linear time for \(m \le 10^5\). Start at a vertex with out-degree one higher than in-degree, else anywhere.

Push edges in postorder and reverse to obtain the arrangement. The code tabs are empty; the reasoning is the Eulerian-path construction.

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