Best Time to Buy and Sell Stock: the idea to remember.

The core insight

Keep the lowest price seen so far and evaluate each later selling price.

Why it works

The cheapest earlier buy is always the best partner for a fixed selling day. This is also a prefix minimum scan.

The mistake to avoid

A buy must precede a sell. Do not use a future minimum.

Commented Java solution

Use java.util.* imports and a surrounding solution class. Tree snippets assume the standard TreeNode definition; linked lists use ListNode and Clone Graph uses a Node with val and neighbors; Copy Random List uses Node with val, next and random. Read the input assumptions above.

int maxProfit(int[] prices) {
    int cheapest = Integer.MAX_VALUE, best = 0;
    for (int price : prices) {
        cheapest = Math.min(cheapest, price);
        best = Math.max(best, price - cheapest);
    }
    return best;
}

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