32 lines
709 B
Ruby
32 lines
709 B
Ruby
|
# Find majority element(if exists) in a given sorted array.
|
||
|
|
# A majority element in an array A[] of size n is an element that appears more than n/2 times
|
||
|
|
#Time-complexity: O(logn), Auxiliary-space:O(1)
|
||
|
|
#Algorithm: Binary-search
|
||
|
|
|
||
|
|
def majority_element(a)
|
||
|
|
len=a.length
|
||
|
|
lo=0
|
||
|
|
hi=len-1
|
||
|
|
candidate=a[lo+(hi-lo)/2]
|
||
|
|
while hi-lo>1
|
||
|
|
mid=lo+(hi-lo)/2
|
||
|
|
if a[mid]>=candidate
|
||
|
|
hi=mid
|
||
|
|
else
|
||
|
|
lo=mid
|
||
|
|
end
|
||
|
|
end
|
||
|
|
|
||
|
|
if a[hi]==candidate
|
||
|
|
left_index=hi
|
||
|
|
right_index=left_index+(len/2)
|
||
|
|
if a[right_index]==candidate
|
||
|
|
return candidate
|
||
|
|
else
|
||
|
|
return -1
|
||
|
|
end
|
||
|
|
end
|
||
|
|
|
||
|
|
|
||
|
|
end
|
||
|
|
majority_element([1,2, 2, 3, 3, 3, 3]) # => 3
|