Find the smallest positive integer value that cannot be represented as sum of any subset of a given array

Question Details:

Given a sorted array (sorted in non-decreasing order) of positive numbers, find the smallest positive integer value that cannot be represented as sum of elements of any subset of given set. 
Expected time complexity is O(n).

Examples:

Input:  arr[] = {1, 3, 6, 10, 11, 15};
Output: 2

Input:  arr[] = {1, 1, 1, 1};
Output: 5

Input:  arr[] = {1, 1, 3, 4};
Output: 10

Input:  arr[] = {1, 2, 5, 10, 20, 40};
Output: 4

Input:  arr[] = {1, 2, 3, 4, 5, 6};
Output: 22

more: http://www.geeksforgeeks.org/find-smallest-value-represented-sum-subset-given-array/

Solution Explanation:
http://stackoverflow.com/a/21078133/4507251