feat: solve No.2196

Author: BaffinLee

2101-2200/2196. Create Binary Tree From Descriptions.md

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+# 2196. Create Binary Tree From Descriptions
+
+- Difficulty: Medium.
+- Related Topics: Array, Hash Table, Tree, Depth-First Search, Breadth-First Search, Binary Tree.
+- Similar Questions: Convert Sorted List to Binary Search Tree, Number Of Ways To Reconstruct A Tree.
+
+## Problem
+
+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:
+
+![](https://assets.leetcode.com/uploads/2022/02/09/example1drawio.png)
+
+```
+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:
+
+![](https://assets.leetcode.com/uploads/2022/02/09/example2drawio.png)
+
+```
+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
+
+```javascript
+/**
+ * Definition for a binary tree node.
+ * function TreeNode(val, left, right) {
+ *     this.val = (val===undefined ? 0 : val)
+ *     this.left = (left===undefined ? null : left)
+ *     this.right = (right===undefined ? null : right)
+ * }
+ */
+/**
+ * @param {number[][]} descriptions
+ * @return {TreeNode}
+ */
+var createBinaryTree = function(descriptions) {
+    var nodeMap = {};
+    var hasNoParentMap = {};
+    for (var i = 0; i < descriptions.length; i++) {
+        var [parent, child, isLeft] = descriptions[i];
+        if (hasNoParentMap[parent] === undefined) hasNoParentMap[parent] = true;
+        hasNoParentMap[child] = false;
+        if (!nodeMap[parent]) nodeMap[parent] = new TreeNode(parent);
+        if (!nodeMap[child]) nodeMap[child] = new TreeNode(child);
+        if (isLeft) {
+            nodeMap[parent].left = nodeMap[child];
+        } else {
+            nodeMap[parent].right = nodeMap[child];
+        }
+    }
+    return nodeMap[Object.keys(hasNoParentMap).filter(key => hasNoParentMap[key])[0]];
+};
+```
+
+**Explain:**
+
+nope.
+
+**Complexity:**
+
+* Time complexity : O(n).
+* Space complexity : O(n).