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#29 Count Pairs That Fill the Row

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A layout tool places two cards side by side in a row that is target pixels wide. The card widths are in widths.

Write countFillingPairs(widths, target) that returns how many pairs of indexes i < j have widths[i] + widths[j] === target. Cards with the same width are still different cards.

Example 1

Input: widths = [3,5,2,5,3], target = 8
Output: 4

Each of the two 3s pairs with each of the two 5s: 4 pairs.

Example 2

Input: widths = [4,4,4,4], target = 8
Output: 6

Any two of the four 4s work, and there are 6 ways to pick two.

Constraints

  • 0 <= widths.length <= 10^5
  • -10^9 <= widths[i], target <= 10^9
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Run tries your code on the examples. Submit runs every test, including hidden edge cases, and solves the problem when they all pass.

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