[20190415] easy problems

Author: iamminji

README.md

@@ -36,6 +36,7 @@
 | [0103](https://leetcode.com/problems/binary-tree-zigzag-level-order-traversal/description/)  | Binary Tree Zigzag Level Order Traversal  | medium     | python         |           |
 | [0107](https://leetcode.com/problems/binary-tree-level-order-traversal-ii/description/)      | Binary Tree Level Order Traversal II      | easy       | python         |           |
 | [0111](https://leetcode.com/problems/minimum-depth-of-binary-tree/)      |  Minimum Depth of Binary Tree      | easy       | [python](🙂easy/0111.Minimum-Depth-of-Binary-Tree/minimum_depth_of_binary_tree.py)         |    tree       |
+| [0121](https://leetcode.com/problems/best-time-to-buy-and-sell-stock/)         | Best Time to Buy and Sell Stock        | easy       | [python](🙂easy/0121.Best-Time-to-Buy-and-Sell-Stock/best_time_to_buy_and_sell_stock.py)         |           |
 | [0122](https://leetcode.com/problems/best-time-to-buy-and-sell-stock-ii/description)         | Best Time to Buy and Sell Stock II        | easy       | python         |           |
 | [0139](https://leetcode.com/problems/word-break/description/)                                | Word Break                                | medium     | python         |           |
 | [0141](https://leetcode.com/problems/linked-list-cycle/description)                          | Linked List Cycle                         | easy       | python         |           |

🙂easy/0121.Best-Time-to-Buy-and-Sell-Stock/best_time_to_buy_and_sell_stock.py

@@ -0,0 +1,29 @@
+# 121. Best Time to Buy and Sell Stock
+# https://leetcode.com/problems/best-time-to-buy-and-sell-stock/
+
+
+from typing import List
+
+
+class Solution:
+    # prices 라는 integer list 가 들어올 때 가장 적은 가격에 사서 비싸게 팔 때 생기는 이익을 리턴하는 문제다.
+    def maxProfit(self, prices: List[int]) -> int:
+
+        # 최소 금액 buy 와 현재 금액 - buy 의 값의 maximum 을 구하면 된다.
+        # 왜냐하면 가장 적은 가격에 사서 (min) 가장 비싸게 팔아야 (max) 하기 때문이다.
+        # greedy 로 생각해서 하면 된다.
+
+        buy = float("inf")
+        res = 0
+        for price in prices:
+            # 최소 금액 갱신
+            buy = min(buy, price)
+            # 판매 최대 이익
+            res = max(res, price - buy)
+
+        return res
+
+
+if __name__ == '__main__':
+    sol = Solution()
+    assert sol.maxProfit([7, 1, 5, 3, 6, 4]) == 5