diff --git a/31. Next Permutation/31. Next Permutation.csproj b/31. Next Permutation/31. Next Permutation.csproj
new file mode 100644
index 0000000..dfe3535
--- /dev/null
+++ b/31. Next Permutation/31. Next Permutation.csproj	
@@ -0,0 +1,11 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net10.0</TargetFramework>
+    <RootNamespace>_31._Next_Permutation</RootNamespace>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+</Project>
diff --git a/31. Next Permutation/Program.cs b/31. Next Permutation/Program.cs
new file mode 100644
index 0000000..69128ca
--- /dev/null
+++ b/31. Next Permutation/Program.cs	
@@ -0,0 +1,30 @@
+var sol = new Solution();
+
+var cases = new (int[] input, int[] expected)[]
+{
+	(new[] { 1, 2, 3 }, new[] { 1, 3, 2 }),
+	(new[] { 3, 2, 1 }, new[] { 1, 2, 3 }),
+	(new[] { 1, 1, 5 }, new[] { 1, 5, 1 })
+};
+
+foreach (var (input, expected) in cases)
+{
+	var nums = (int[])input.Clone();
+	sol.NextPermutation(nums);
+	var status = AreEqual(nums, expected) ? "ok" : "fail";
+	Console.WriteLine($"{Format(input)} -> {Format(nums)} (expected: {Format(expected)}) {status}");
+}
+
+static string Format(int[] nums) => $"[{string.Join(",", nums)}]";
+
+static bool AreEqual(int[] left, int[] right)
+{
+	if (left.Length != right.Length)
+		return false;
+	for (var i = 0; i < left.Length; i++)
+	{
+		if (left[i] != right[i])
+			return false;
+	}
+	return true;
+}
diff --git a/31. Next Permutation/Solution.cs b/31. Next Permutation/Solution.cs
new file mode 100644
index 0000000..1fbb09b
--- /dev/null
+++ b/31. Next Permutation/Solution.cs	
@@ -0,0 +1,17 @@
+public class Solution
+{
+    public void NextPermutation(int[] nums)
+    {
+        var i = nums.Length - 2;
+        while(i >= 0 && nums[i] >= nums[i+1])
+            i--;
+        if(i >= 0)
+        {
+            var j = nums.Length - 1;
+            while(nums[i] >= nums[j])
+                j--;
+            (nums[i], nums[j]) = (nums[j], nums[i]);
+        }
+        Array.Reverse(nums, i+1, nums.Length-i-1);
+    }
+}
\ No newline at end of file
diff --git a/Leetcode.sln b/Leetcode.sln
index 3193445..ea36914 100644
--- a/Leetcode.sln
+++ b/Leetcode.sln
@@ -189,6 +189,8 @@ Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "23. Merge k Sorted Lists",
 EndProject
 Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "26. Remove Duplicates from Sorted Array", "26. Remove Duplicates from Sorted Array\26. Remove Duplicates from Sorted Array.csproj", "{5CE01D6E-7C12-45DC-8C26-DFE67F16AD3F}"
 EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "31. Next Permutation", "31. Next Permutation\31. Next Permutation.csproj", "{DC76DEDE-AF9D-429B-B53B-204E89E90EA0}"
+EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
 		Debug|Any CPU = Debug|Any CPU
@@ -1315,6 +1317,18 @@ Global
 		{5CE01D6E-7C12-45DC-8C26-DFE67F16AD3F}.Release|x64.Build.0 = Release|Any CPU
 		{5CE01D6E-7C12-45DC-8C26-DFE67F16AD3F}.Release|x86.ActiveCfg = Release|Any CPU
 		{5CE01D6E-7C12-45DC-8C26-DFE67F16AD3F}.Release|x86.Build.0 = Release|Any CPU
+		{DC76DEDE-AF9D-429B-B53B-204E89E90EA0}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{DC76DEDE-AF9D-429B-B53B-204E89E90EA0}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{DC76DEDE-AF9D-429B-B53B-204E89E90EA0}.Debug|x64.ActiveCfg = Debug|Any CPU
+		{DC76DEDE-AF9D-429B-B53B-204E89E90EA0}.Debug|x64.Build.0 = Debug|Any CPU
+		{DC76DEDE-AF9D-429B-B53B-204E89E90EA0}.Debug|x86.ActiveCfg = Debug|Any CPU
+		{DC76DEDE-AF9D-429B-B53B-204E89E90EA0}.Debug|x86.Build.0 = Debug|Any CPU
+		{DC76DEDE-AF9D-429B-B53B-204E89E90EA0}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{DC76DEDE-AF9D-429B-B53B-204E89E90EA0}.Release|Any CPU.Build.0 = Release|Any CPU
+		{DC76DEDE-AF9D-429B-B53B-204E89E90EA0}.Release|x64.ActiveCfg = Release|Any CPU
+		{DC76DEDE-AF9D-429B-B53B-204E89E90EA0}.Release|x64.Build.0 = Release|Any CPU
+		{DC76DEDE-AF9D-429B-B53B-204E89E90EA0}.Release|x86.ActiveCfg = Release|Any CPU
+		{DC76DEDE-AF9D-429B-B53B-204E89E90EA0}.Release|x86.Build.0 = Release|Any CPU
 	EndGlobalSection
 	GlobalSection(SolutionProperties) = preSolution
 		HideSolutionNode = FALSE