From cc475a8b16b0e9323623c6532e114dceeb64353a Mon Sep 17 00:00:00 2001
From: chai <chaifix@163.com>
Date: Mon, 21 Jun 2021 15:53:42 +0800
Subject: +recursions

---
 Assets/Algorithms/Recursion.cs | 63 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 63 insertions(+)
 create mode 100644 Assets/Algorithms/Recursion.cs

(limited to 'Assets/Algorithms/Recursion.cs')

diff --git a/Assets/Algorithms/Recursion.cs b/Assets/Algorithms/Recursion.cs
new file mode 100644
index 0000000..30ebd8a
--- /dev/null
+++ b/Assets/Algorithms/Recursion.cs
@@ -0,0 +1,63 @@
+using System;
+using System.Collections;
+using System.Collections.Generic;
+using AlgorithmCollection;
+
+// 递归与分治算法
+
+namespace AlgorithmCollection.Recursion
+{
+
+    public static class RecursionHelper
+    {
+
+        #region 全排列，返回数据的所有组合
+        public static void Permutations<T>(List<T> data, ref List<List<T>> perms)
+        {
+            if (perms == null)
+                perms = new List<List<T>>();
+            foreach(List<T> perm in perms)
+            {
+                List<T> p = new List<T>(perm);
+                perms.Add(p);
+            }
+        }
+
+        // 生成器形式，每次返回一个组合
+        public static IEnumerable Permutations<T>(List<T> data)
+        {
+            foreach(var perm in _Permutations(data, 0, data.Count - 1))
+            {
+                yield return perm;
+            }
+        }
+
+        // 计算start~end范围内的全排列
+        private static IEnumerable _Permutations<T>(List<T> data, int start , int end, List<T> perm = null)
+        {
+            if(perm == null)
+                perm = new List<T>(data.Count);
+            if(start == end)
+            {
+                perm.Add(data[start]);
+                yield return perm;
+                perm.RemoveAt(perm.Count - 1);
+            }
+            else
+            {
+                for (int i = start; i <= end; ++i)
+                {
+                    perm.Add(data[i]);
+                    Algorithms.Swap(ref data, start, i);
+                    IEnumerator itor = _Permutations(data, start + 1, end, perm).GetEnumerator();
+                    while(itor.MoveNext())
+                        yield return itor.Current;
+                    Algorithms.Swap(ref data, start, i);
+                    perm.RemoveAt(perm.Count - 1);
+                }
+            }
+        }
+        #endregion  
+
+    }
+}
\ No newline at end of file
-- 
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