permutation

Author: tanfanhua

src/com/blankj/medium/_0060/Solution.java

@@ -0,0 +1,53 @@
+package com.blankj.medium._060;
+
+import java.util.ArrayList;
+import java.util.List;
+
+public class Solution {
+
+    public static void main(String[] args) {
+        Solution s = new Solution();
+        System.out.println(s.permutationSequence(4, 9));
+    }
+
+    public String permutationSequence(int n, int k) {
+        if (n <= 0 || k <= 0) {
+            return "";
+        }
+
+        List<String> elements = new ArrayList<>();
+        for (int i = 1; i <= n; i++) {
+            elements.add(String.valueOf(i));
+        }
+        List<String> res = dfsPermutation(elements);
+        if (k > res.size()) {
+            throw new IllegalArgumentException("k is bigger than permutation list size");
+        }
+
+        return res.get(k);
+    }
+
+    public List<String> dfsPermutation(List<String> elements) {
+        List<String> res = new ArrayList<>();
+        if (elements.size() == 1) {
+            res.add(elements.get(0));
+        } else {
+            for (String e : elements) {
+                List<String> left = new ArrayList<>();
+                for (String l : elements) {
+                    if (l != e) {
+                        left.add(l);
+                    }
+                }
+                List<String> subsequences = dfsPermutation(left);
+                for (String sub : subsequences) {
+                    res.add(e + sub);
+                }
+            }
+        }
+
+        return res;
+    }
+
+
+}