add 3211

Author: fishercoder1534

paginated_contents/algorithms/4th_thousand/README.md

@@ -1,5 +1,6 @@
 | #    | Title                                                                                                                                                               | Solutions                                                                                                                         | Video                                                                         | Difficulty | Tag                                                                  
 |------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------|-------------------------------------------------------------------------------|------------|----------------------------------------------------------------------
+| 3211 | [Generate Binary Strings Without Adjacent Zeros](https://leetcode.com/problems/generate-binary-strings-without-adjacent-zeros/)                                                                                       | [Java](https://github.com/fishercoder1534/Leetcode/blob/master/src/main/java/com/fishercoder/solutions/fourththousand/_3211.java) |                                                                               | Medium     | Recursion, Backtracking
 | 3210 | [Find the Encrypted String](https://leetcode.com/problems/find-the-encrypted-string/)                                                                                       | [Java](https://github.com/fishercoder1534/Leetcode/blob/master/src/main/java/com/fishercoder/solutions/fourththousand/_3210.java) |                                                                               | Easy       |
 | 3208 | [Alternating Groups II](https://leetcode.com/problems/alternating-groups-ii/)                                                                                       | [Java](https://github.com/fishercoder1534/Leetcode/blob/master/src/main/java/com/fishercoder/solutions/fourththousand/_3208.java) |                                                                               | Medium     |
 | 3206 | [Alternating Groups I](https://leetcode.com/problems/alternating-groups-i/)                                                                                         | [Java](https://github.com/fishercoder1534/Leetcode/blob/master/src/main/java/com/fishercoder/solutions/fourththousand/_3206.java) |                                                                               | Easy       |

src/main/java/com/fishercoder/solutions/fourththousand/_3211.java

@@ -0,0 +1,67 @@
+package com.fishercoder.solutions.fourththousand;
+
+import java.util.ArrayList;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Set;
+
+public class _3211 {
+    public static class Solution1 {
+        public List<String> validStrings(int n) {
+            List<String> result = new ArrayList<>();
+            for (int i = n / 2; i <= n; i++) {
+                List<String> combinations = generateCombinations(i, n - i);
+                for (String s : combinations) {
+                    if (noAdjacentZeroes(s)) {
+                        result.add(s);
+                    }
+                }
+            }
+            return result;
+        }
+
+        private boolean noAdjacentZeroes(String s) {
+            for (int i = 0; i < s.length() - 1; i++) {
+                if (s.charAt(i) == '0' && s.charAt(i + 1) == '0') {
+                    return false;
+                }
+            }
+            return true;
+        }
+
+        private List<String> generateCombinations(int ones, int zeroes) {
+            int[] nums = new int[ones + zeroes];
+            int i = 0;
+            while (ones-- > 0) {
+                nums[i++] = 1;
+            }
+            return permuteUnique(nums);
+        }
+
+        private List<String> permuteUnique(int[] nums) {
+            Set<String> set = new HashSet<>();
+            set.add("");
+            set = recurse(nums, set, 0);
+            List<String> list = new ArrayList<>();
+            for (String s : set) {
+                list.add(s);
+            }
+            return list;
+        }
+
+        private Set<String> recurse(int[] nums, Set<String> set, int pos) {
+            if (pos == nums.length) {
+                return set;
+            }
+            Set<String> newSet = new HashSet<>();
+            for (String s : set) {
+                for (int i = 0; i <= s.length(); i++) {
+                    StringBuilder sb = new StringBuilder(s);
+                    sb.insert(i, nums[pos]);
+                    newSet.add(sb.toString());
+                }
+            }
+            return recurse(nums, newSet, pos + 1);
+        }
+    }
+}