Added number-zoo-patrol.rb

Author: malachispencer

ruby/6kyu/number-zoo-patrol.rb

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+=begin
+Codewars. 28/04/20. 'Number Zoo Patrol'. 6kyu. Here we create a method that 
+takes an array of numbers from 1 to n, with one missing element, and returns 
+the missing number. The missing number is not necessarily in between the numbers 
+in the array, it can be before position [0] or after position [-1]. Here is the 
+solution I developed to solve the challenge.
+1) We define our method find_missing_number_ms, which takes nums, an array of
+ numbers from 1 to n - with one number missing - as its argument.
+2) We create a range from 1 to the size of nums plus 1, then sum this range, we
+ sum nums, then we subtract the difference, the difference is the missing
+ number.
+3) This algorithm is doable because nums always starts at 1 and the gap
+ between values is always 1, so the last value in nums - with the missing
+ number placed in - will always be the size without the missing + 1.
+=end
+
+def find_missing_number_ms(nums)
+ (1..nums.size+1).sum - nums.sum
+end
+
+=begin
+Here is another algorithm that can be used to solve the challenge.
+1) If nums were [2,3,4], n would be 4. n plus 1 would be 5, divided by 2 would
+ be 2.5. n multiplied by 2.5 would be 10. nums.sum would be 9. So 10 minus 9
+ would be 1 - the missing number.
+2) If nums were [1,2,3,4,5,6,7,8], n would be 9. 9 plus 1 would be 10, divided
+ by 2 would be 5. n multiplied by 5 would be 45. nums.sum would be 36. So 45
+ minus 36 would give us the missing number, 9.
+=end
+
+def find_missing_number(nums)
+ n = nums.size + 1 ; n * (n + 1) / 2 - nums.sum
+end