Merge pull request #897 from 0xff-dev/1922

Add solution and test-cases for problem 1922

Author: 6boris

leetcode/1901-2000/1922.Count-Good-Numbers/README.md

@@ -1,28 +1,35 @@
 # [1922.Count Good Numbers][title]
 
-> [!WARNING|style:flat]
-> This question is temporarily unanswered if you have good ideas. Welcome to [Create Pull Request PR](https://github.com/kylesliu/awesome-golang-algorithm)
-
 ## Description
+A digit string is **good** if the digits **(0-indexed)** at **even** indices are **even** and the digits at **odd** indices are **prime** (`2`, `3`, `5`, or `7`).
+
+- For example, `"2582"` is good because the digits (`2` and `8`) at even positions are even and the digits (`5` and `2`) at odd positions are prime. However, `"3245"` is **not** good because `3` is at an even index but is not even.
+
+Given an integer `n`, return the **total** number of good digit strings of length `n`. Since the answer may be large, **return it modulo** `10^9 + 7`.
+
+A **digit string** is a string consisting of digits `0` through `9` that may contain leading zeros.
 
 **Example 1:**
 
 ```
-Input: a = "11", b = "1"
-Output: "100"
+Input: n = 1
+Output: 5
+Explanation: The good numbers of length 1 are "0", "2", "4", "6", "8".
 ```
 
-## 题意
-> ...
-
-## 题解
+**Example 2:**
 
-### 思路1
-> ...
-Count Good Numbers
-```go
 ```
+Input: n = 4
+Output: 400
+```
+
+**Example 3:**
 
+```
+Input: n = 50
+Output: 564908303
+```
 
 ## 结语
 

leetcode/1901-2000/1922.Count-Good-Numbers/Solution.go

@@ -1,5 +1,28 @@
 package Solution
 
-func Solution(x bool) bool {
-	return x
+const mod1922 = 1000000007
+
+func quickPow(a int, n int64) int {
+	if n == 0 {
+		return 1
+	}
+	if n == 1 {
+		return a
+	}
+
+	ans := quickPow(a, n>>1)
+	ans = (ans * ans) % mod1922
+	if n&1 == 1 {
+		ans = (ans * a) % mod1922
+	}
+	return ans
+}
+
+func Solution(n int64) int {
+	add := int64(0)
+	if n&1 == 1 {
+		add = 1
+	}
+	mid := n >> 1
+	return (quickPow(5, mid+add) * quickPow(4, mid)) % mod1922
 }

leetcode/1901-2000/1922.Count-Good-Numbers/Solution_test.go

@@ -10,12 +10,12 @@ func TestSolution(t *testing.T) {
 	//	测试用例
 	cases := []struct {
 		name   string
-		inputs bool
-		expect bool
+		inputs int64
+		expect int
 	}{
-		{"TestCase", true, true},
-		{"TestCase", true, true},
-		{"TestCase", false, false},
+		{"TestCase1", 1, 5},
+		{"TestCase2", 4, 400},
+		{"TestCase3", 50, 564908303},
 	}
 
 	//	开始测试
@@ -30,10 +30,10 @@ func TestSolution(t *testing.T) {
 	}
 }
 
-//	压力测试
+// 压力测试
 func BenchmarkSolution(b *testing.B) {
 }
 
-//	使用案列
+// 使用案列
 func ExampleSolution() {
 }