Merge pull request #1031 from zhangjiongwx/master

添加 0344.反转字符串、0541.反转字符串II、0015.三数之和、0018.四数之和 C#版本

problems/0015.三数之和.md

@@ -545,5 +545,64 @@ int** threeSum(int* nums, int numsSize, int* returnSize, int** returnColumnSizes
 }
 ```
 
+C#:
+```csharp
+public class Solution
+{
+    public IList<IList<int>> ThreeSum(int[] nums)
+    {
+        var result = new List<IList<int>>();
+
+        Array.Sort(nums);
+
+        for (int i = 0; i < nums.Length - 2; i++)
+        {
+            int n1 = nums[i];
+
+            if (n1 > 0)
+                break;
+
+            if (i > 0 && n1 == nums[i - 1])
+                continue;
+
+            int left = i + 1;
+            int right = nums.Length - 1;
+
+            while (left < right)
+            {
+                int n2 = nums[left];
+                int n3 = nums[right];
+                int sum = n1 + n2 + n3;
+
+                if (sum > 0)
+                {
+                    right--;
+                }
+                else if (sum < 0)
+                {
+                    left++;
+                }
+                else
+                {
+                    result.Add(new List<int> { n1, n2, n3 });
+
+                    while (left < right && nums[left] == n2)
+                    {
+                        left++;
+                    }
+
+                    while (left < right && nums[right] == n3)
+                    {
+                        right--;
+                    }
+                }
+            }
+        }
+
+        return result;
+    }
+}
+```
+
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 <div align="center"><img src=https://code-thinking.cdn.bcebos.com/pics/01二维码一.jpg width=500> </img></div>

problems/0018.四数之和.md

@@ -445,5 +445,67 @@ func fourSum(_ nums: [Int], _ target: Int) -> [[Int]] {
 }
 ```
 
+C#:
+```csharp
+public class Solution
+{
+    public IList<IList<int>> FourSum(int[] nums, int target)
+    {
+        var result = new List<IList<int>>();
+
+        Array.Sort(nums);
+
+        for (int i = 0; i < nums.Length - 3; i++)
+        {
+            int n1 = nums[i];
+            if (i > 0 && n1 == nums[i - 1])
+                continue;
+
+            for (int j = i + 1; j < nums.Length - 2; j++)
+            {
+                int n2 = nums[j];
+                if (j > i + 1 && n2 == nums[j - 1])
+                    continue;
+
+                int left = j + 1;
+                int right = nums.Length - 1;
+
+                while (left < right)
+                {
+                    int n3 = nums[left];
+                    int n4 = nums[right];
+                    int sum = n1 + n2 + n3 + n4;
+
+                    if (sum > target)
+                    {
+                        right--;
+                    }
+                    else if (sum < target)
+                    {
+                        left++;
+                    }
+                    else
+                    {
+                        result.Add(new List<int> { n1, n2, n3, n4 });
+
+                        while (left < right && nums[left] == n3)
+                        {
+                            left++;
+                        }
+
+                        while (left < right && nums[right] == n4)
+                        {
+                            right--;
+                        }
+                    }
+                }
+            }
+        }
+
+        return result;
+    }
+}
+```
+
 -----------------------
 <div align="center"><img src=https://code-thinking.cdn.bcebos.com/pics/01二维码一.jpg width=500> </img></div>

problems/0344.反转字符串.md

@@ -253,6 +253,19 @@ void reverseString(char* s, int sSize){
 }
 ```
 
+C#：
+```csharp
+public class Solution
+{
+    public void ReverseString(char[] s)
+    {
+        for (int i = 0, j = s.Length - 1; i < j; i++, j--)
+        {
+            (s[i], s[j]) = (s[j], s[i]);
+        }
+    }
+}
+```
 
 -----------------------
 <div align="center"><img src=https://code-thinking.cdn.bcebos.com/pics/01二维码一.jpg width=500> </img></div>

problems/0541.反转字符串II.md

@@ -294,9 +294,21 @@ func reverseStr(_ s: String, _ k: Int) -> String {
 }
 ```
 
-
-
-
+C#：
+```csharp
+public class Solution
+{
+    public string ReverseStr(string s, int k)
+    {
+        Span<char> span = s.ToCharArray().AsSpan();
+        for (int i = 0; i < span.Length; i += 2 * k)
+        {
+            span[i + k < span.Length ? i..(i + k) : i..].Reverse();
+        }
+        return span.ToString();
+    }
+}
+```
 
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 <div align="center"><img src=https://code-thinking.cdn.bcebos.com/pics/01二维码一.jpg width=500> </img></div>