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花花酱 LeetCode 1640. Check Array Formation Through Concatenation

You are given an array of distinct integers arr and an array of integer arrays pieces, where the integers in pieces are distinct. Your goal is to form arr by concatenating the arrays in pieces in any order. However, you are not allowed to reorder the integers in each array pieces[i].

Return true if it is possible to form the array arr from pieces. Otherwise, return false.

Example 1:

Input: arr = [85], pieces = [[85]]
Output: true

Example 2:

Input: arr = [15,88], pieces = [[88],[15]]
Output: true
Explanation: Concatenate [15] then [88]

Example 3:

Input: arr = [49,18,16], pieces = [[16,18,49]]
Output: false
Explanation: Even though the numbers match, we cannot reorder pieces[0].

Example 4:

Input: arr = [91,4,64,78], pieces = [[78],[4,64],[91]]
Output: true
Explanation: Concatenate [91] then [4,64] then [78]

Example 5:

Input: arr = [1,3,5,7], pieces = [[2,4,6,8]]
Output: false


  • 1 <= pieces.length <= arr.length <= 100
  • sum(pieces[i].length) == arr.length
  • 1 <= pieces[i].length <= arr.length
  • 1 <= arr[i], pieces[i][j] <= 100
  • The integers in arr are distinct.
  • The integers in pieces are distinct (i.e., If we flatten pieces in a 1D array, all the integers in this array are distinct).

Solution: Hashtable

Store the index of the first number of each piece, for each number a in arr, concat the entire piece array whose first element equals to a.

Time complexity: O(n)
Space complexity: O(n)


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