Construct String from Binary Tree

Problem

Given the root of a binary tree, construct a string consisting of parenthesis and integers from a binary tree with the preorder traversal way, and return it.

Omit all the empty parenthesis pairs that do not affect the one-to-one mapping relationship between the string and the original binary tree.

Constraints

• The number of nodes in the tree is in the range [1, 104].
• -1000 <= Node.val <= 1000

Solution

The problem Construct String from Binary Tree can be solved by recursively constructing string from child nodes based on the problem description.

Implementation

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution
{
  public:
    string tree2str(TreeNode *root)
    {
        if (root == NULL)
            return "";

        string ret = to_string(root->val);
        if (root->left != NULL || root->right != NULL)
            ret += "(" + tree2str(root->left) + ")";
        if (root->right != NULL)
            ret += "(" + tree2str(root->right) + ")";

        return ret;
    }
};