LeetCode Entry

2610. Convert an Array Into a 2D Array With Conditions

2.01.2024 medium 2024 kotlin

Convert numbers array into array of unique number-rows.

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Problem TLDR

Convert numbers array into array of unique number-rows.

Intuition

Let’s count each kind of number, then use each unique number to build the rows.

Approach

Kotlin’s API can be helpful:

• groupBy
• mapValues
• buildList

Complexity

• Time complexity: \(O(uf)\) where, u - number of uniq elements, f - max frequency. Worst case O(n^2): 1 2 3 4 1 1 1 1, u = n / 2, f = n / 2. This can be improved to O(n) by removing the empty collections from freq.

• Space complexity: \(O(n)\)

Code

  fun findMatrix(nums: IntArray): List<List<Int>> {
    val freq = nums.groupBy { it }
      .mapValues { it.value.toMutableList() }
    return buildList {
      repeat(freq.values.maxOf { it.size }) {
        add(buildList {
          for ((k, v) in freq)
            if (v.isNotEmpty()) add(v.removeLast())
        })
      }
    }
  }