LeetCode-in-Java

2196. Create Binary Tree From Descriptions

Medium

You are given a 2D integer array descriptions where descriptions[i] = [parenti, childi, isLefti] indicates that parenti is the parent of childi in a binary tree of unique values. Furthermore,

• If isLefti == 1, then childi is the left child of parenti.
• If isLefti == 0, then childi is the right child of parenti.

Construct the binary tree described by descriptions and return its root.

The test cases will be generated such that the binary tree is valid.

Example 1:

Input: descriptions = [[20,15,1],[20,17,0],[50,20,1],[50,80,0],[80,19,1]]

Output: [50,20,80,15,17,19]

Explanation: The root node is the node with value 50 since it has no parent.

The resulting binary tree is shown in the diagram.

Example 2:

Input: descriptions = [[1,2,1],[2,3,0],[3,4,1]]

Output: [1,2,null,null,3,4]

Explanation: The root node is the node with value 1 since it has no parent.

The resulting binary tree is shown in the diagram.

Constraints:

• 1 <= descriptions.length <= 104
• descriptions[i].length == 3
• 1 <= parenti, childi <= 105
• 0 <= isLefti <= 1
• The binary tree described by descriptions is valid.

Solution

import com_github_leetcode.TreeNode;
import java.util.HashMap;
import java.util.Map;

/*
 * 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 TreeNode createBinaryTree(int[][] descriptions) {
        Map<Integer, Data> map = new HashMap<>();
        for (int[] description : descriptions) {
            Data data = map.get(description[0]);
            if (data == null) {
                data = new Data();
                data.node = new TreeNode(description[0]);
                data.isHead = true;
                map.put(description[0], data);
            }
            Data childData = map.get(description[1]);
            if (childData == null) {
                childData = new Data();
                childData.node = new TreeNode(description[1]);
                map.put(childData.node.val, childData);
            }
            childData.isHead = false;
            if (description[2] == 1) {
                data.node.left = childData.node;
            } else {
                data.node.right = childData.node;
            }
        }
        for (Map.Entry<Integer, Data> entry : map.entrySet()) {
            if (entry.getValue().isHead) {
                return entry.getValue().node;
            }
        }
        return null;
    }

    private static class Data {
        TreeNode node;
        boolean isHead;
    }
}

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