#15 Easy Time O(n) Space O(1) Animated

Best Time to Buy and Sell Stock

Choose a single day to buy and a later day to sell a stock for maximum profit.

LeetCode ↗

Tutor Mode Build the mental model before you memorize the code.

1 Predict
2 Trace
3 Explain
4 Implement

Pattern Sliding window with a maintained constraint

Expand to include new information, then shrink only until the window is valid again.

Approach: One Pass — Track Cheapest Buy

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Core Insight

Expand to include new information, then shrink only until the window is valid again.

Common Pitfall

Update the best answer only when the window satisfies the problem constraint.

Description

Choose a single day to buy and a later day to sell a stock for maximum profit.

Constraints

Time Complexity: O(n)
Space Complexity: O(1)

Solution — One Pass — Track Cheapest Buy

TimeO(n) SpaceO(1)

Implementation Plan

Translate the invariant into these small, testable moves.

Expand the right edge.
Update window state.
Shrink from the left while invalid.
Record the best valid window.

def maxProfit(prices):
    buy = prices[0]
    best = 0
    for price in prices[1:]:
        buy = min(buy, price)
        best = max(best, price - buy)
    return best

Longest Substring Without Repeating Characters

Best Time to Buy and Sell Stock

Steps

Description

Constraints

Tags

Solution — One Pass — Track Cheapest Buy