Project Euler Problem 42

問題

The n'th term of the sequence of triangle numbers is given by, t(n) = 1/2 n(n+1);
so the first ten triangle numbers are:

1, 3, 6, 10, 15, 21, 28, 36, 45, 55, ...

By converting each letter in a word to a number corresponding to its alphabetical
position and adding these values we form a word value. For example, the word value
for SKY is 19 + 11 + 25 = 55 = t(10).
If the word value is a triangle number then we shall call the word a triangle word.

Using words.txt (right click and 'Save Link/Target As...'), a 16K text file containing
nearly two-thousand common English words, how many are triangle words?

ソース

$triangles = Array.new(200, 0)
(1..20).inject(0) do |n, i|
	$triangles[n + i] = n + i
end

File::open("words.txt") do |f|
	puts f.gets.split(",").map{|n|
		n.gsub(/\"/, "").bytes.to_a.map{|b| b - 64}.inject(:+)
	}.inject(0){|s, n|
		s + ($triangles[n].zero? ? 0 : 1)
	}
end

解答

162

感想

あらかじめ三角数を計算しておけば、後は計算するだけです。