Given an array of Integers, where arr[i] represents range of coverage for router i.
If arr[i]=k, it means this router will cover range from (i-k) to (i+k).
Calculate the minimum numbers of routers required to cover full range of array.
Eg.
arr = [1,1,2,1,1] , output = 1 ( arr[2] covers all the range).
Expected time complexity O(n)