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Java Solution for LeetCode algorithm problems, continually updating

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package g0601_0700.s0606_construct_string_from_binary_tree;

// #Easy #String #Depth_First_Search #Tree #Binary_Tree
// #2022_03_21_Time_1_ms_(100.00%)_Space_42.3_MB_(91.51%)

import com_github_leetcode.TreeNode;

/*
 * 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 "";
        }
        StringBuilder sb = new StringBuilder();
        preorder(t, sb);
        return sb.toString();
    }

    private void preorder(TreeNode root, StringBuilder sb) {
        if (root == null) {
            return;
        }
        sb.append(root.val);
        if (root.left != null) {
            sb.append("(");
            preorder(root.left, sb);
            sb.append(")");
        }
        if (root.right != null) {
            if (root.left == null) {
                sb.append("()");
            }
            sb.append("(");
            preorder(root.right, sb);
            sb.append(")");
        }
    }
}