0606 - Construct String from Binary Tree (Easy)

Input: root = [1,2,3,4]
Output: "1(2(4))(3)"
Explanation: Originally, 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: root = [1,2,3,null,4]
Output: "1(2()(4))(3)"
Explanation: Almost the same as the first example, except we cannot omit the first parenthesis pair to break the one-to-one mapping relationship between the input and the output.

/**
 * 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) {}
 * };
 */

// Time Complexity: O(N) where N is the number of the nodes in the tree
// Space Complexity: O(H) where H is the height of the tree.
// In worse case, H can be N when it is a left skewed binary tree / right skewed binary tree
class Solution {
public:
    // case 1: root is nullptr -> ""
    // case 2: root doesn't have left sub tree and right sub tree -> root->val
    // case 3: root->left is not nullptr -> root->val + (dfs result from left sub tree)
    // case 4: root->left is nullptr but root->right is not nullptr -> root->val + ()
    // case 5: root->right is not nullptr -> root->val + (dfs result from right sub tree)
    string tree2str(TreeNode* root) {
        // handle case 1
        if (!root) return "";
        // we convert root->val to string here, then append results from different cases
        string s = to_string(root->val);
        // handle case 2
        // this line is obviously not necessary
        if (!root->left && !root->right) s += "";
        // handle case 3
        if (root->left) s += "(" + tree2str(root->left) + ")";
        // handle case 4
        // alternatively, you can use `else if (root->right) s += "()";`
        if (!root->left && root->right) s += "()";
        // handle case 5
        if (root->right) s += "(" + tree2str(root->right) + ")";
        return s;
    }
};

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right

# Time Complexity: O(N) where N is the number of the nodes in the tree
# Space Complexity: O(H) where H is the height of the tree.
# In worse case, H can be N when it is a left skewed binary tree / right skewed binary tree
class Solution:
    # case 1: root is nullptr -> ""
    # case 2: root doesn't have left sub tree and right sub tree -> root.val
    # case 3: root.left is not nullptr -> root.val + (dfs result from left sub tree)
    # case 4: root.left is nullptr but root.right is not nullptr -> root.val + ()
    # case 5: root.right is not nullptr -> root.val + (dfs result from right sub tree)
    def tree2str(self, root: Optional[TreeNode]) -> str:
        # handle case 1
        if root is None:
            return ''
        # we convert root.val to string here, then append results from different cases
        s = str(root.val)
        # handle case 2
        # this line is obviously not necessary
        if root.left is None and root.right is None:
            s += ''
        # handle case 3
        if root.left:
            s += '({})'.format(self.tree2str(root.left))
        # handle case 4
        # alternatively, you can use `elif root.right: s += '()'`
        if root.left is None and root.right:
            s += '()'
        # handle case 5
        if root.right:
            s += '({})'.format(self.tree2str(root.right))
        return s

public class Solution {
    public string Tree2str(TreeNode root) {
        if(root == null) return null;

        string left = Tree2str(root.left);
        string right = Tree2str(root.right);

        string result = "";

        if(left == null && right == null) {
            result = $"{root.val}";
        }
        else if(left != null && right == null) {
            result = $"{root.val}({left})";
        }
        else {
            result = $"{root.val}({left})({right})";
        }

        return result;
    }
}

defmodule Solution do
  # Enumerate the cases:
  @spec tree2str(root :: TreeNode.t | nil) :: String.t
  def tree2str(%TreeNode{val: v, left: nil, right: nil}), do:
    "#{v}"

  def tree2str(%TreeNode{val: v, left: lch, right: nil}), do:
    "#{v}(#{tree2str(lch)})"

  def tree2str(%TreeNode{val: v, left: nil, right: rch}), do:
    "#{v}()(#{tree2str(rch)})"

  def tree2str(%TreeNode{val: v, left: lch, right: rch}), do:
    "#{v}(#{tree2str(lch)})(#{tree2str(rch)})"
end

/**
 * 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;
 *     }
 * }
 */
class Solution {
    /**Approach
     * Access the elements in preorder dfs as we usually do and
     * a simple add-on is the parenthesis for each child.
     * Add the parenthesis to right child if:
     * 1. right child is present [the case when both children exist]
     * 2. left is null [the case when only right child exist]
     * Note: parenthesis will not be added if child doesn't exist,
     * why? -> 'cause null wouldn't be appended to our string
     */
    StringBuilder sb = new StringBuilder();
    public String tree2str(TreeNode root) {
        f(root);
        return sb.toString();
    }
    private void f(TreeNode root) {
        if (root == null) return;
        sb.append(root.val);
        if (root.left == null && root.right == null) return;
        sb.append("(");
        f(root.left);
        sb.append(")");
        // Add the parenthesis to right child if
        // 1. right child is present [the case when both children exist]
        // 2. left is null [the case when only right child exist]
        // Note: parenthesis will not be added if child doesn't exist,
        // why? -> 'cause null wouldn't be appended to our string
        if (root.left == null || root.right != null) {
            sb.append("(");
        }
        f(root.right);
        if (root.left == null || root.right != null) {
            sb.append(")");
        }
    }
}