Fruit Into Baskets

Problem

You are visiting a farm that has a single row of fruit trees. Each tree produces fruit of type fruits[i]. You only have two baskets, and each basket can only hold a single type of fruit. Return the maximum number of fruits you can pick.

Examples

The brute-force approach

Try every possible starting tree. From each starting position, extend as far right as you can while keeping at most 2 distinct fruit types. Track the maximum length seen.

max_fruits = 0

for i in range(len(fruits)):
    basket = {}
    for j in range(i, len(fruits)):
        basket[fruits[j]] = basket.get(fruits[j], 0) + 1
        if len(basket) > 2:
            break               # ← had to stop, too many types
        max_fruits = max(max_fruits, j - i + 1)

return max_fruits

For each of the n starting positions you may scan up to n more trees. O(n²) total. The key observation is that when you advance the left boundary to kick out a fruit type, you don't need to start fresh from scratch. A sliding window maintains the right boundary and left boundary simultaneously, advancing each only forward.

Spotting the pattern

This is a Sliding Window problem. The key question to ask yourself:

How do I find the longest contiguous stretch of trees that uses at most 2 fruit types without restarting from scratch each time?

Answering that is where it clicks, and it's exactly what the guided walkthrough below builds with you: the pattern reasoning, a progressive hint ladder that never spoils the answer, a row-by-row dry run, the optimized solution, and an in-browser editor to run your code against real test cases.