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。