Add solution for 951 (#49)

Author: havanagrawal

README.md

@@ -50,6 +50,7 @@ Your ideas/fixes/algorithms are more than welcome!
 |965|[Univalued Binary Tree](https://leetcode.com/problems/univalued-binary-tree/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_965.java) | O(n) | O(h) | |Easy| DFS, recursion|
 |961|[N-Repeated Element in Size 2N Array](https://leetcode.com/problems/n-repeated-element-in-size-2n-array/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_961.java) | O(n) | O(1) | |Easy|
 |953|[Verifying an Alien Dictionary](https://leetcode.com/problems/verifying-an-alien-dictionary/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_953.java) | O(1) | O(1) | |Easy|
+|951|[Flip Equivalent Binary Trees](https://leetcode.com/problems/flip-equivalent-binary-trees/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_951.java) | O(n) | O(h) | |Medium| Tree, DFS, recursion|
 |950|[Reveal Cards In Increasing Order](https://leetcode.com/problems/reveal-cards-in-increasing-order/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_950.java) | O(nlogn) | O(n) | |Medium|
 |944|[Delete Columns to Make Sorted](https://leetcode.com/problems/delete-columns-to-make-sorted/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_944.java) | O(n) | O(1) | |Easy|
 |942|[DI String Match](https://leetcode.com/problems/di-string-match/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_942.java) | O(n) | O(n) | |Easy|

src/main/java/com/fishercoder/solutions/_951.java

@@ -0,0 +1,33 @@
+package com.fishercoder.solutions;
+
+import com.fishercoder.common.classes.TreeNode;
+
+/**
+ * 951. Flip Equivalent Binary Trees
+ *
+ * For a binary tree T, we can define a flip operation as follows: choose any node, and swap the left and right child subtrees.
+ *
+ * A binary tree X is flip equivalent to a binary tree Y if and only if we can make X equal to Y after some number of flip operations.
+ *
+ * Write a function that determines whether two binary trees are flip equivalent.  The trees are given by root nodes root1 and root2.
+ *
+ * Note:
+ *  * Each tree will have at most 100 nodes.
+ *  * Each value in each tree will be a unique integer in the range [0, 99].
+ */
+
+public class _951 {
+    public static class Solution1 {
+        public boolean flipEquiv(TreeNode root1, TreeNode root2) {
+            if (root1 == null && root2 == null) return true;
+            if (root1 == null || root2 == null) return false;
+
+            if (root1.val != root2.val) return false;
+
+            return (
+                (flipEquiv(root1.left, root2.left) && flipEquiv(root1.right, root2.right)) ||
+                (flipEquiv(root1.left, root2.right) && flipEquiv(root1.right, root2.left))
+            );
+        }
+    }
+}

src/test/java/com/fishercoder/_951Test.java

@@ -0,0 +1,35 @@
+package com.fishercoder;
+
+import com.fishercoder.common.classes.TreeNode;
+import com.fishercoder.common.utils.TreeUtils;
+import com.fishercoder.solutions._951;
+import org.junit.BeforeClass;
+import org.junit.Test;
+
+import java.util.Arrays;
+
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
+
+public class _951Test {
+    private static _951.Solution1 solution1;
+
+    @BeforeClass
+    public static void setup() {
+        solution1 = new _951.Solution1();
+    }
+
+    @Test
+    public void test1() {
+        TreeNode root1 = TreeUtils.constructBinaryTree(Arrays.asList(1, 2, 3, 4, 5, 6, null, null, null, 7, 8));
+        TreeNode root2 = TreeUtils.constructBinaryTree(Arrays.asList(1, 3, 2, null, 6, 4, 5, null, null, null, null, 8, 7));
+        assertTrue(solution1.flipEquiv(root1, root2));
+    }
+
+    @Test
+    public void test2() {
+        TreeNode root1 = TreeUtils.constructBinaryTree(Arrays.asList(1, 2, 3));
+        TreeNode root2 = TreeUtils.constructBinaryTree(Arrays.asList(1, 2, 4));
+        assertFalse(solution1.flipEquiv(root1, root2));
+    }
+}