Solution: Find Symmetric Pairs in an Array

Let’s solve the Find Symmetric Pairs in an Array problem.

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Given an array of pairs, nums, find all the symmetric pairsBy definition, (a, b) and (c, d) are symmetric pairs iff, a = d and b = c. from it. If no symmetric pair is found, return an empty array.


  • 22 \leq nums.length 103\leq10^3

  • nums[i].length ==2== 2

  • 105-10^5\leq nums[i][1] 105\leq 10^5

  • 105-10^5\leq nums[i][2] 105\leq10^5


In this solution, we use an unordered set to store the encountered pairs and check whether the symmetric pairs exist.

Here are the steps of the algorithm:

  1. Initialize an empty unordered set, lookup, to store encountered pairs, and an empty array, result, to store symmetric pairs.

  2. Iterate through nums, and for each pair, check if the reverse of it exists in lookup.

    1. If the reverse exists, append the current pair and its reverse to the result array.

    2. Otherwise, add the current pair to lookup.

  3. After iterating through nums, return the result containing symmetric pairs.

Let’s look at the illustration below to better understand the solution:

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