patching-array.py

# Time:  O(s + logn), s is the number of elements in the array
# Space: O(1)

# Given a sorted positive integer array nums and
# an integer n, add/patch elements to the array
# such that any number in range [1, n] inclusive
# can be formed by the sum of some elements in the
# array. Return the minimum number of patches required.
#
# Example 1:
# nums = [1, 3], n = 6
# Return 1.
#
# Combinations of nums are [1], [3], [1,3], which form 
# possible sums of: 1, 3, 4.
# Now if we add/patch 2 to nums, the combinations are: 
# [1], [2], [3], [1,3], [2,3], [1,2,3].
# Possible sums are 1, 2, 3, 4, 5, 6, which now covers
# the range [1, 6].
# So we only need 1 patch.
#
# Example 2:
# nums = [1, 5, 10], n = 20
# Return 2.
# The two patches can be [2, 4].
#
# Example 3:
# nums = [1, 2, 2], n = 5
# Return 0.


class Solution(object):
    def minPatches(self, nums, n):
        """
        :type nums: List[int]
        :type n: int
        :rtype: int
        """
        patch, miss, i = 0, 1, 0
        while miss <= n:
            if i < len(nums) and nums[i] <= miss:
                miss += nums[i]
                i += 1
            else:
                miss += miss
                patch += 1

        return patch