feat: add solution

solution/2900-2999/2978.Symmetric Coordinates/README.md

@@ -61,6 +61,10 @@ The output table is sorted by X and Y in ascending order.
 
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+**方法一：窗口函数 + 自连接**
+
+我们可以使用窗口函数 `ROW_NUMBER()` 来为每一行添加一个自增的序号，然后再自连接两张表，连接条件为 `p1.x = p2.y AND p1.y = p2.x AND p1.x <= p1.y AND p1.id != p2.id`，最后再排序去重即可。
+
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 ### **SQL**

solution/2900-2999/2978.Symmetric Coordinates/README_EN.md

@@ -57,6 +57,10 @@ The output table is sorted by X and Y in ascending order.
 
 ## Solutions
 
+**Solution 1: Window Function + Self Join**
+
+We can use the window function `ROW_NUMBER()` to add an auto-incrementing sequence number to each row. Then, we perform a self join on the two tables, with the join conditions being `p1.x = p2.y AND p1.y = p2.x AND p1.x <= p1.y AND p1.id != p2.id`. Finally, we sort and remove duplicates.
+
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 ### **SQL**

solution/2900-2999/2979.Most Expensive Item That Can Not Be Bought/README.md

@@ -42,6 +42,12 @@
 
 <!-- 这里可写通用的实现逻辑 -->
 
+**方法一：Chicken McNugget 定理**
+
+根据 Chicken McNugget 定理，两个互质的正整数 $a$ 和 $b$，最大不能表示的数为 $ab - a - b$。
+
+时间复杂度 $O(1)$，空间复杂度 $O(1)$。
+
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 ### **Python3**

solution/2900-2999/2979.Most Expensive Item That Can Not Be Bought/README_EN.md

@@ -38,6 +38,12 @@
 
 ## Solutions
 
+**Solution 1: Chicken McNugget Theorem**
+
+According to the Chicken McNugget Theorem, for two coprime positive integers $a$ and $b$, the largest number that cannot be expressed as a combination of $a$ and $b$ is $ab - a - b$.
+
+The time complexity is $O(1)$, and the space complexity is $O(1)$.
+
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 ### **Python3**