[LeetCode] Construct String from Binary Tree

Problem: You need to construct a string consists of parenthesis and integers from a binary tree with the preorder traversing way.

The null node needs to be represented by empty parenthesis pair "()". And you need to omit all the empty parenthesis pairs that don't affect the one-to-one mapping relationship between the string and the original binary tree.

Example:

Input: Binary tree: [1,2,3,4]
       1
     /   \
    2     3
   /    
  4     

Output: "1(2(4))(3)"

Explanation: Originallay it needs to be "1(2(4)())(3()())", 
but you need to omit all the unnecessary empty parenthesis pairs. 
And it will be "1(2(4))(3)".

Input: Binary tree: [1,2,3,null,4]
       1
     /   \
    2     3
     \  
      4 

Output: "1(2()(4))(3)"

Explanation: Almost the same as the first example, 
except we can't omit the first parenthesis pair to break the one-to-one mapping relationship between the input and the output.

Approach: We can use post-order traversal here. We just need to handle the special condition where left subtree is null and right subtree is not.

Have a look at implementation to understand the approach. It's fairly easy to understand.

 

Implementation in C#:

    public string Tree2str(TreeNode node) 

    {

        if (node == null)

        {

            return string.Empty;

        }

        string leftStr = this.Tree2str(node.left);

        string rightStr = this.Tree2str(node.right);

        string result = node.val.ToString();

        if (leftStr == string.Empty && rightStr == string.Empty)

        {

            return result;

        }        

        result += $"({leftStr})";

        if (rightStr != string.Empty)

        {

            result += $"({rightStr})";

        }

        return result;

    }

Complexity: O(n) with assumption that string addition takes constant time.