add a solution for 114

Author: fishercoder1534

src/main/java/com/fishercoder/solutions/_114.java

@@ -20,4 +20,31 @@ public void flatten(TreeNode root) {
             }
         }
     }
+
+    public static class Solution2 {
+
+        /**
+         * Credit: https://leetcode.com/problems/flatten-binary-tree-to-linked-list/solution/
+         */
+        public void flatten(TreeNode root) {
+            flat(root);
+        }
+
+        private TreeNode flat(TreeNode curr) {
+            if (curr == null) {
+                return null;
+            }
+            if (curr.left == null && curr.right == null) {
+                return curr;
+            }
+            TreeNode leftTail = flat(curr.left);
+            TreeNode rightTail = flat(curr.right);
+            if (leftTail != null) {
+                leftTail.right = curr.right;
+                curr.right = curr.left;
+                curr.left = null;
+            }
+            return rightTail == null ? leftTail : rightTail;
+        }
+    }
 }