feat(sliding-window): add 121_best_time_to_buy_and_sell_stock.rs

Author: whisperpine

README.md

@@ -117,3 +117,9 @@ so that you can use `just tf TEST` command to test.
 | - | - | - |
 | 704. Binary Search | Easy | [704_binary_search.rs](./tests/704_binary_search.rs) |
 | 74. Search a 2D Matrix | Medium | [74_search_a_2d_matrix.rs](./tests/74_search_a_2d_matrix.rs) |
+
+### Sliding Window
+
+| Problem | Difficulty | Solution |
+| - | - | - |
+| 121. Best Time to Buy and Sell Stock | Easy | [121_best_time_to_buy_and_sell_stock.rs](./tests/121_best_time_to_buy_and_sell_stock.rs) |

tests/121_best_time_to_buy_and_sell_stock.rs

@@ -0,0 +1,31 @@
+// 121. Best Time to Buy and Sell Stock
+// https://leetcode.com/problems/best-time-to-buy-and-sell-stock/description/
+// Topics: Sliding Window.
+// Difficulty: Easy.
+
+#[test]
+fn test_121_best_time_to_buy_and_sell_stock() {}
+
+#[derive(Debug)]
+pub struct Solution;
+
+// ---------------------------------
+// copy to leetcode starts from here
+// ---------------------------------
+
+impl Solution {
+    pub fn max_profit(prices: Vec<i32>) -> i32 {
+        let mut buy = prices[0];
+        let mut profit = 0;
+        for &item in prices.iter().skip(1) {
+            if item < buy {
+                buy = item;
+                continue;
+            }
+            if item - buy > profit {
+                profit = item - buy;
+            }
+        }
+        profit
+    }
+}