add permutation

data/1.Java初阶/9.控制执行流程/5.函数/config.json

@@ -7,6 +7,7 @@
   "children": [],
   "export": [
     "call.json",
-    "parser.json"
+    "parser.json",
+    "permutation.json"
   ]
 }
\ No newline at end of file

data/1.Java初阶/9.控制执行流程/5.函数/permutation.json

+{
+  "type": "code_options",
+  "author": "刘鑫",
+  "source": "permutation.md",
+  "notebook_enable": false,
+  "exercise_id": "a52d29ac72cf48669824894ba7f7427a"
+}
\ No newline at end of file

data/1.Java初阶/9.控制执行流程/5.函数/permutation.md

+# 排列
+
+给定一个几十个字符内的短字符串，简单起见我们假设这个字符串没有重复字符，那么生成其全排列的代码是：
+
+## 答案
+
+```java
+package net.csdn;
+
+import java.util.ArrayList;
+import java.util.List;
+
+public class Main {
+
+    public static String drop(String token, int idx){
+        if(idx==0){
+            return token.substring(1);
+        }
+        if(idx>=token.length()-1){
+            return token.substring(0, idx);
+        }
+        return token.substring(0, idx) + token.substring(idx+1);
+    }
+
+    public static List<String> permutation(String token){
+        if(token.length() < 2){
+            return List.of(token);
+        }
+        if(token.length()==2){
+            String item = token.charAt(1) + String.valueOf(token.charAt(0));
+            return List.of(token, item);
+        }
+        List<String> result = new ArrayList<>();
+        for(int idx=0;idx<token.length();idx++){
+            char chr = token.charAt(idx);
+            String next = drop(token, idx);
+
+            for(var rest: permutation(next)){
+                result.add(chr +rest);
+            }
+        }
+        return result;
+    }
+
+    public static void main(String[] args) {
+        String token = args[0];
+        for(var item: permutation(token)){
+            System.out.println(item);
+        }
+    }
+}
+```
+
+## 选项
+
+### 方法定义不对
+
+```java
+package net.csdn;
+
+import java.util.ArrayList;
+import java.util.List;
+
+public class Main {
+
+    public String drop(String token, int idx) {
+        if (idx == 0) {
+            return token.substring(1);
+        }
+        if (idx >= token.length() - 1) {
+            return token.substring(0, idx);
+        }
+        return token.substring(0, idx) + token.substring(idx + 1);
+    }
+
+    public List<String> permutation(String token) {
+        if (token.length() < 2) {
+            return List.of(token);
+        }
+        if (token.length() == 2) {
+            String item = token.charAt(1) + String.valueOf(token.charAt(0));
+            return List.of(token, item);
+        }
+        List<String> result = new ArrayList<>();
+        for (int idx = 0; idx < token.length(); idx++) {
+            char chr = token.charAt(idx);
+            String next = drop(token, idx);
+
+            for (var rest : permutation(next)) {
+                result.add(chr + rest);
+            }
+        }
+        return result;
+    }
+
+    public static void main(String[] args) {
+        String token = args[0];
+        for (var item : permutation(token)) {
+            System.out.println(item);
+        }
+    }
+}
+```
+
+### 方法引用不对
+
+```java
+package net.csdn;
+
+import java.util.ArrayList;
+import java.util.List;
+
+public class Main {
+
+    public static String drop(String token, int idx){
+        if(idx==0){
+            return token.substring(1);
+        }
+        if(idx>=token.length()-1){
+            return token.substring(0, idx);
+        }
+        return token.substring(0, idx) + token.substring(idx+1);
+    }
+
+    public static List<String> permutation(String token){
+        if(token.length() < 2){
+            return List.of(token);
+        }
+        if(token.length()==2){
+            String item = token.charAt(1) + String.valueOf(token.charAt(0));
+            return List.of(token, item);
+        }
+        List<String> result = new ArrayList<>();
+        for(int idx=0;idx<token.length();idx++){
+            char chr = token.charAt(idx);
+            String next = drop(token, idx);
+
+            for(var rest: permutation(next)){
+                result.add(chr +rest);
+            }
+        }
+        return result;
+    }
+
+    public static void main(String[] args) {
+        String token = args[0];
+        for(var item: this.permutation(token)){
+            System.out.println(item);
+        }
+    }
+}
+```
+
+### 方法定义不对
+
+```java
+package net.csdn;
+
+import java.util.ArrayList;
+import java.util.List;
+
+public class Main {
+
+    String drop(String token, int idx){
+        if(idx==0){
+            return token.substring(1);
+        }
+        if(idx>=token.length()-1){
+            return token.substring(0, idx);
+        }
+        return token.substring(0, idx) + token.substring(idx+1);
+    }
+
+    List<String> permutation(String token){
+        if(token.length() < 2){
+            return List.of(token);
+        }
+        if(token.length()==2){
+            String item = token.charAt(1) + String.valueOf(token.charAt(0));
+            return List.of(token, item);
+        }
+        List<String> result = new ArrayList<>();
+        for(int idx=0;idx<token.length();idx++){
+            char chr = token.charAt(idx);
+            String next = drop(token, idx);
+
+            for(var rest: permutation(next)){
+                result.add(chr +rest);
+            }
+        }
+        return result;
+    }
+
+    public static void main(String[] args) {
+        String token = args[0];
+        for(var item: permutation(token)){
+            System.out.println(item);
+        }
+    }
+}
+```
+
+### D
+
+全都是错的，Java 函数不能递归调用
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