LeetCode 121: Best Time to Buy and Sell Stock.

🚀 Cracking LeetCode 121: Best Time to Buy and Sell Stock.

🔗 Link: leetcode.com/problems/best-time-to-buy-and-sell-stock
(Search: LC 121)

⚠️ The Common Mistake:
Most people start with brute force:

Try every buy-sell pair → O(n²)

It works… but doesn’t scale.

🔍 Did you catch the optimization?

Instead of checking all combinations, track the minimum price seen so far.

At each step:
profit = current price - min so far

This converts the problem into a single pass solution.

💡 The Optimal Approach:

int buy = prices[0];

for each price:
→ update minimum (buy)
→ calculate profit
→ update maxProfit

No nested loops. No extra space.

🔥 The Interviewer’s Edge (Java Logic):

You’re not just finding profit.

You’re maintaining:
👉 “best buying opportunity till now”

This is a classic greedy pattern:
Make the best decision using past information — in one pass.

🧠 Pattern Recognition:

Whenever you see:

maximize profit
order matters (buy before sell)

👉 Think:
“track minimum so far”

Expert Cheat Sheet:
✅ TC: O(n) — Single traversal
✅ SC: O(1) — Constant space

Key Takeaway:
Brute force finds answers.
Greedy thinking finds scalable solutions.

Brute force ➔ Smart decisions ➔ One pass.

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Видео LeetCode 121: Best Time to Buy and Sell Stock. канала DesignAndAlgo