Construct String from Binary Tree

Leetcode/ConstructStringFromBinaryTree.java

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+/**
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     int val;
+ *     TreeNode left;
+ *     TreeNode right;
+ *     TreeNode() {}
+ *     TreeNode(int val) { this.val = val; }
+ *     TreeNode(int val, TreeNode left, TreeNode right) {
+ *         this.val = val;
+ *         this.left = left;
+ *         this.right = right;
+ *     }
+ * }
+ */
+public class Solution {
+    public String tree2str(TreeNode t) {
+        if (t == null) return "";
+
+        String result = t.val + "";
+
+        String left = tree2str(t.left);
+        String right = tree2str(t.right);
+
+        if (left == "" && right == "") return result;
+        if (left == "") return result + "()" + "(" + right + ")";
+        if (right == "") return result + "(" + left + ")";
+        return result + "(" + left + ")" + "(" + right + ")";
+    }
+}