Add solution and test-cases for problem 493

Author: 0xff-dev

leetcode/401-500/0493.Reverse-Pairs/README.md

@@ -1,28 +1,34 @@
 # [493.Reverse Pairs][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
+Given an integer array `nums`, return the number of **reverse pairs** in the array.
+
+A **reverse pair** is a pair `(i, j)` where:
+
+- `0 <= i < j < nums.length` and
+- `nums[i] > 2 * nums[j]`.
+
 
 **Example 1:**
 
 ```
-Input: a = "11", b = "1"
-Output: "100"
+Input: nums = [1,3,2,3,1]
+Output: 2
+Explanation: The reverse pairs are:
+(1, 4) --> nums[1] = 3, nums[4] = 1, 3 > 2 * 1
+(3, 4) --> nums[3] = 3, nums[4] = 1, 3 > 2 * 1
 ```
 
-## 题意
-> ...
+**Example 2:**
 
-## 题解
-
-### 思路1
-> ...
-Reverse Pairs
-```go
 ```
-
+Input: nums = [2,4,3,5,1]
+Output: 3
+Explanation: The reverse pairs are:
+(1, 4) --> nums[1] = 4, nums[4] = 1, 4 > 2 * 1
+(2, 4) --> nums[2] = 3, nums[4] = 1, 3 > 2 * 1
+(3, 4) --> nums[3] = 5, nums[4] = 1, 5 > 2 * 1
+```
 
 ## 结语
 

leetcode/401-500/0493.Reverse-Pairs/Solution.go

@@ -1,5 +1,46 @@
 package Solution
 
-func Solution(x bool) bool {
-	return x
+func merge(nums []int, start, end int) int {
+	mid := start + (end-start)/2
+	l, r := start, mid+1
+	tmpNums := make([]int, 0)
+	count := 0
+	for ; l <= mid; l++ {
+		for ; r <= end && nums[l] > 2*nums[r]; r++ {
+
+		}
+		count += r - 1 - mid
+	}
+	l, r = start, mid+1
+	for l <= mid && r <= end {
+		if nums[l] < nums[r] {
+			tmpNums = append(tmpNums, nums[l])
+			l++
+		} else {
+			tmpNums = append(tmpNums, nums[r])
+			r++
+		}
+	}
+	for ; l <= mid; l++ {
+		tmpNums = append(tmpNums, nums[l])
+	}
+	for ; r <= end; r++ {
+		tmpNums = append(tmpNums, nums[r])
+	}
+	for k := 0; k < len(tmpNums); k++ {
+		nums[k+start] = tmpNums[k]
+	}
+	return count
+}
+
+func MergeSort(nums []int, start, end int) int {
+	if start < end {
+		mid := start + (end-start)/2
+		return MergeSort(nums, start, mid) + MergeSort(nums, mid+1, end) + merge(nums, start, end)
+	}
+	return 0
+}
+
+func Solution(nums []int) int {
+	return MergeSort(nums, 0, len(nums)-1)
 }

leetcode/401-500/0493.Reverse-Pairs/Solution_test.go

@@ -10,12 +10,11 @@ func TestSolution(t *testing.T) {
 	//	测试用例
 	cases := []struct {
 		name   string
-		inputs bool
-		expect bool
+		inputs []int
+		expect int
 	}{
-		{"TestCase", true, true},
-		{"TestCase", true, true},
-		{"TestCase", false, false},
+		{"TestCase1", []int{1, 3, 2, 3, 1}, 2},
+		{"TestCase2", []int{2, 4, 3, 5, 1}, 3},
 	}
 
 	//	开始测试
@@ -30,10 +29,10 @@ 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() {
 }