feat(problems): add solution for Best Time to Buy and Sell Stock

work/Easy/Array/121.best-time-to-buy-and-sell-stock.py

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+"""
+121. Best Time to Buy and Sell Stock
+Difficulty: Easy
+https://leetcode.com/problems/best-time-to-buy-and-sell-stock/
+
+──────────────────────────────────────────────────
+
+You are given an array prices where prices[i] is the price of a given
+stock on the i^th day.
+
+You want to maximize your profit by choosing a single day to buy one
+stock and choosing a different day in the future to sell that stock.
+
+Return the maximum profit you can achieve from this transaction. If
+you cannot achieve any profit, return 0.
+
+ 
+
+Example 1:
+
+Input: prices = [7,1,5,3,6,4]
+Output: 5
+Explanation: Buy on day 2 (price = 1) and sell on day 5 (price = 6),
+profit = 6-1 = 5.
+Note that buying on day 2 and selling on day 1 is not allowed because
+you must buy before you sell.
+
+Example 2:
+
+Input: prices = [7,6,4,3,1]
+Output: 0
+Explanation: In this case, no transactions are done and the max
+profit = 0.
+
+ 
+
+Constraints:
+
+	• 1 <= prices.length <= 10^5
+
+	• 0 <= prices[i] <= 10^4
+"""
+
+class Solution:
+    def maxProfit(self, prices: List[int]) -> int:
+        left = min(prices)
+        for right in range(len(prices)):
+            while curr > left:
+                curr -= prices[left]
+                left += 1
+            ans = max(ans, curr)
+        return ans