Counting number of variables in an array not passing test cases - arrays

Question

So I am doing a program on Hackerrank.
The problem is
You have been asked to help study the population of birds migrating across the continent. Each type of bird you are interested in will be identified by an integer value. Each time a particular kind of bird is spotted, its id number will be added to your array of sightings. You would like to be able to find out which type of bird is most common given a list of sightings. Your task is to print the type number of that bird and if two or more types of birds are equally common, choose the type with the smallest ID number.
For example, assume your bird sightings are of types . There are two each of types and , and one sighting of type . Pick the lower of the two types seen twice: type .
Function Description
Complete the migratoryBirds function in the editor below. It should return the lowest type number of the most frequently sighted bird.
migratoryBirds has the following parameter(s):
arr: an array of integers representing types of birds sighted
Input Format
The first line contains an integer denoting , the number of birds sighted and reported in the array .
The second line describes as space-separated integers representing the type numbers of each bird sighted.
Constraints
It is guaranteed that each type is , , , , or .
Output Format
Print the type number of the most common bird; if two or more types of birds are equally common, choose the type with the smallest ID number.
This is my code. It is passing all but one of the test cases. Can you help?
x = []
for i in range (0,len(arr)):
x.append(arr.count(arr[i]))
for i in range(0,len(x)):
if x[i] == max(x):
out = i
return out

Answer 1

I don't see that your code passes any test cases, even with the indentation fixed. Your input size can be as large as 105, so that pretty much eliminates any O(n2) solutions that require traversing the entire list, which is unnecessary. The culprit here is arr.count(), which traverses the entire list for every element. You probably meant to traverse the list for every bird type, which results in 5 traversals of the input list:
x = []
for i in range(1, 6):
x.append(arr.count(i))
out = 0
best = x[0]
for i in range(len(x)):
if x[i] > best:
best = x[i]
out = i
return out + 1
The important restriction to note in this problem is that there are only 5 bird types (numbered 1-5), so we can initialize a counts array of length 6 with a dummy element at position 0 so indices map to counts neatly. Then, we can iterate through our input bird type list in one pass, updating our counter array. Lastly, take the max index of the counter array. The "smallest id for ties" restriction is automatically handled by the way max() works--it'll take the smallest index automatically. Here's the code:
counts = [-1, 0, 0, 0, 0, 0]
for bird in arr:
counts[bird] += 1
return counts.index(max(counts))
Note that the variable names are descriptive. This helps ensure that the purpose of each variable is clear and reduces bugs and erroneous assumptions about the program state.