Best Time to Buy And Sell Stock

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Problem

Given an array where prices[i] is the price of a stock on day i, choose a single day to buy and a later day to sell to maximize your profit, and return the maximum profit achievable. Return 0 if no profit is possible.

Examples

Example 1

Input:prices = [7,1,5,3,6,4]

Output:5

Buy on day 2 (price 1), sell on day 5 (price 6).

Example 2

Input:prices = [7,6,4,3,1]

Output:0

Prices only fall, so no profit is possible.

Constraints

1 <= prices.length <= 10^5

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How to approach it: the Sliding Window pattern

A window — a contiguous subarray or substring — expands and shrinks as it moves across the input, so you track a running condition instead of recomputing it from scratch for every possible window.

Look for this pattern when

The problem asks for the longest/shortest/best contiguous subarray or substring meeting a condition.
Brute force would recompute a sum or count for every possible window — a sign that work can be shared between overlapping ranges.

Read the full Sliding Window guide →

Video walkthroughs

Original problem on LeetCode ↗