Frequency Counting

The Idea

A surprising number of questions collapse into counting: the most common item, the first value that repeats, whether two collections hold the same multiset. Build a map from value to count in one pass, then read the answer off the map. Rescanning the list per item would cost O(n²).

Real-World Example

A dawn bird survey volunteer walks a fixed route with one notebook line per species. Every sighting is a tick beside an existing name, never a fresh page. When the route ends the totals are already finished, and no stretch of path is ever walked twice.

The Code

def top_item(items):
    counts = {}
    for item in items:
        # first sighting starts at 0, every later one adds a tick
        counts[item] = counts.get(item, 0) + 1
    return max(counts, key=counts.get)     # highest tally wins

birds = ["robin", "crow", "robin", "wren", "crow", "robin"]
print(top_item(birds))          # robin
print(len(set(birds)))          # 3 distinct species