permutation sequence

Author: zgpeace

code/LeetCode/src/backtracking/PermutationSequence.java

@@ -0,0 +1,50 @@
+package backtracking;
+
+import java.util.ArrayList;
+import java.util.List;
+
+// https://leetcode.com/problems/permutation-sequence/
+public class PermutationSequence {
+
+  public static void main(String[] args) {
+    PermutationSequence obj = new PermutationSequence();
+    String result = obj.getPermutation(4, 9);
+    System.out.println("result > " + result);
+  }
+
+  public String getPermutation(int n, int k) {
+    // factorial
+    int[] factorial = new int[n + 1];
+    factorial[0] = 1;
+    int sum = 1;
+
+    // create an array of factorial lookup
+    for (int i = 1; i <= n; i++) {
+      sum *= i;
+      // factorial[] = {1, 1, 2, 6, 24, ..., n!}
+      factorial[i] = sum;
+    }
+
+    // create a list of numbers to get indices
+    List<Integer> items = new ArrayList<Integer>();
+    for (int i = 1; i <= n; i++) {
+      // numbers = {1, 2, 3, 4}
+      items.add(i);
+    }
+
+    // i from 0, so k - 1
+    k--;
+
+    StringBuilder sb = new StringBuilder();
+    // calculate
+    for (int i = 1; i <= n; i++) {
+      int index = k / factorial[n - i];
+      int item = items.get(index);
+      sb.append(String.valueOf(item));
+      items.remove(index);
+      k -= index * factorial[n - i];
+    }
+
+    return sb.toString();
+  }
+}