Update solution 047

Author: yanglbme

README.md

@@ -9,7 +9,6 @@
 
 <h2 align="center">LeetCode Solution</h2>
 
-## Introduction
 Complete solutions to Leetcode problems, updated daily.
 
 ## Solution List

solution/047.Permutations II/README.md

@@ -19,6 +19,8 @@
 
 注意：第 i 个数字与第 j 个数字交换时，要求[i, j) 中没有与第 j 个数字相等的数。
 
+解法①：
+
 ```java
 class Solution {
     public List<List<Integer>> permuteUnique(int[] nums) {
@@ -66,4 +68,59 @@ class Solution {
         nums[j] = t;
     }
 }
+```
+
+解法②：
+
+```java
+class Solution {
+    public List<List<Integer>> permuteUnique(int[] nums) {
+        List<List<Integer>> list = new ArrayList<>();
+        permute(list, nums, 0);
+        return list;
+    }
+    
+    private void permute(List<List<Integer>> list, int[] nums, int start) {
+        int end = nums.length - 1;
+        if (start == end) {
+            List<Integer> tmp = new ArrayList<>();
+            for (int val : nums) {
+                tmp.add(val);
+            }
+            
+            list.add(tmp);
+            
+        }
+        
+        
+        int max = Integer.MIN_VALUE;
+        int min = Integer.MAX_VALUE;
+        for (int i = 0; i <= end; ++i) {
+            max = Math.max(max, nums[i]);
+            min = Math.min(min, nums[i]);
+        }
+        // 空间换取时间
+        boolean[] flag = new boolean[max - min + 1];
+        
+        for (int i = start; i <= end; ++i) {
+            int index = nums[i] - min;
+            if (!flag[index]) {
+                // 说明当前的数没在之前出现过
+                swap(nums, i, start);
+                permute(list, nums, start + 1);
+                swap(nums, i, start);
+                
+                // 标志位设为 true
+                flag[index] = true;
+            }
+        }
+        
+    }
+    
+    private void swap(int[] nums, int i, int j) {
+        int t = nums[i];
+        nums[i] = nums[j];
+        nums[j] = t;
+    }
+}
 ```