Minimum Impossible Sum

April 24, 2015

We have today another from our inexhaustible list of interview questions:

Given a list of positive integers, find the smallest number that cannot be calculated as the sum of the integers in the list. For instance, given the integers 4, 13, 2, 3 and 1, the smallest number that cannot be calculated as the sum of the integers in the list is 11.

Your task is to write a program that solves the interview question. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.

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6 Responses to “Minimum Impossible Sum”

  1. mvaneerde said

    Wow, I was *sure* this would require an O(2^n) solution, but the O(n) solution (assuming sorted input) is so elegant.

  2. Paul said

    In Python.

    def impsum(seq):
        runsum = 0
        for e in sorted(seq):
            if e > runsum + 1:
                break
            runsum += e
        return runsum + 1
    
  3. Rutger said

    Finding minimum is easy with sort and a loop of max n iterations.
    Added couple of methods to find them all (within a given range)

    #mis.py
    import random
    import itertools
    
    # The #combinations of a list is 2**(len(list))
    # set param iterations lower than this number and try to be lucky
    # set param iterations higher to increase confidence (but also running time)
    def has_sum_random(n, list_of_positive_integers, iterations):
    	i = 0
    	while i < iterations:
    		i += 1
    		l = list_of_positive_integers[:]
    		total = 0
    		while total < n and l:
    			total += l.pop(random.randrange(len(l)))
    			if total == n:
    				return True
    	return False
    
    
    # tries all combinations of the list to match the sum to n
    def has_sum_deterministic(n, list_of_positive_integers):
    	for i in range(len(list_of_positive_integers)+1):
    		combinations = itertools.combinations(list_of_positive_integers, i)
    		for c in combinations:
    			if sum(c) == n:
    				return True
    	return False
    
    
    def minimum_impossible_sum(list_of_positive_integers):
        s = 1
        for e in sorted(list_of_positive_integers):
            if e > s:
                return s
            s += e
        return s
    
    list_of_positive_integers = [4, 13, 2, 3, 1]
    print minimum_impossible_sum(list_of_positive_integers)
    for n in range(min(list_of_positive_integers), sum(list_of_positive_integers)+1):
    	print n, 
    	print has_sum_random(n, list_of_positive_integers, 2**(len(list_of_positive_integers)+2)), 
    	print has_sum_deterministic(n, list_of_positive_integers)
    
    list_of_positive_integers = range(1, 30)
    print minimum_impossible_sum(list_of_positive_integers)
    for n in range(min(list_of_positive_integers), sum(list_of_positive_integers)+1):
    	print n, 
    	print has_sum_random(n, list_of_positive_integers, 1000), 
    	print has_sum_deterministic(n, list_of_positive_integers)
    

    For [4, 13, 2, 3, 1] it finds
    1 True True
    2 True True
    3 True True
    4 True True
    5 True True
    6 True True
    7 True True
    8 True True
    9 True True
    10 True True
    11 False False
    12 False False
    13 True True
    14 True True
    15 True True
    16 True True
    17 True True
    18 True True
    19 True True
    20 True True
    21 True True
    22 True True
    23 True True

  4. programmingpraxis said

    @mvaneerde: As an exercise, why don’t you write the O(2**n) solution and show it to us. Or if not that, an O(n**2) solution. Or some other time complexity. It is often instructive to be able to compare such solutions.

  5. Jussi Piitulainen said

    I’m not sure if this is O(2^n) or if it’s worse than that. Very roughly, it computes the powerbag of 2^n elements up to 2^n times; the append and map in P may or may not affect the complexity class. What next? Ordering the powerbag by the sum in order to walk it only once?

    (define (conser a) (lambda (d) (cons a d)))

    (define (P B)
      (if (null? B) '(())
          (let ((S (P (cdr B))))
            (append S (map (conser (car B)) S)))))

    (define (min-non-sum B)
      (let s ((PB (P B)) (m 0))
        (if (null? PB) m
            (if (= (apply + (car PB)) m)
                (s (P B) (+ m 1))
                (s (cdr PB) m)))))

    ; (length (P '(3 1 4 1))) => 16
    ; (min-non-sum '(3 1 4 1)) => 10
    ; (min-non-sum '(4 13 2 3 1)) => 11
    ; (min-non-sum '()) => 1
    ; (min-non-sum '(1)) => 2
    ;-)

  6. Globules said

    A Haskell solution. The more efficient version is basically the same as Paul’s Python function.

    import Control.Monad (filterM, liftM)
    import Data.List ((\\), sort)
    import System.Environment (getArgs)
    
    -- Minimum impossible sum.  Brute force version.
    mis' :: [Integer] -> Integer
    mis' =  head . ([1..] \\) . map sum . powerSet
      where powerSet = filterM (const [True, False])
    
    -- Minimum impossible sum.  More efficient version.
    mis :: [Integer] -> Integer
    mis xs = let ys = sort xs
             in head . dropLE ys $ scanl (+) 1 ys
      where dropLE (a:as) (b:bs) | a <= b    = dropLE as bs
                                 | otherwise = b:bs
            dropLE _ bs = bs
    
    main :: IO ()
    main = do
      xs <- liftM (map read) getArgs
      mapM_ (print . mis ) xs
      mapM_ (print . mis') xs
    

    A sample run, printing the output of both functions.

    ./minimpsum "[4, 13, 2, 3, 1]"
    11
    11
    

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