A map keeps a note next to each key

A class election: the teacher reads out the votes, and next to each name on the board makes a tally. That board is a map. Each key (a name) has a value next to it (the tally so far).

A map, also called a dictionary, is a set that stores something next to every key. Finding a key and updating its value both take about one step, thanks to hashing.

Map: a key, with a value stored next to it.

Counting [3, 1, 3, 2, 3]

counts = {3: 1}

Move 1 of 5

Count everything in one walk

Walk the list once. For each value, add 1 to its tally. A value you haven't seen yet has no tally, so treat it as 0 first. When the walk ends, the map holds every value's count.

Then read off the winner. When two values tie, the problem needs a rule to break it: here the smaller value wins.

New key? its count starts at 0.

Ties: need a rule: here the smaller value wins.

In code

counts = {}
for x in nums:
    counts[x] = counts.get(x, 0) + 1
top = max(counts.values())
tied = [x for x in counts
        if counts[x] == top]
return min(tied)