a few tree questions

Author: beck chen

leetcode practice/trees and graphs/binaryTreePostOrderTraversal.js

@@ -0,0 +1,33 @@
+/**
+ * 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 {TreeNode} root
+ * @return {number[]}
+ */
+
+function postorderTraversal(root) {
+    if (!root) { return []; }
+    const results = [];
+
+    const q = [root];
+    while (q.length > 0) {
+        const currentNode = q.pop();
+        results.push(currentNode.val);
+
+        if (currentNode.left) {
+            q.push(currentNode.left);
+        }
+
+        if (currentNode.right) {
+            q.push(currentNode.right);
+        }
+    }
+
+    return results.reverse();
+}

leetcode practice/trees and graphs/binaryTreeUpsideDown.js

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+
+/*
+    class TreeNode {
+        constructor(val) {
+            this.val = val;
+            this.left_ptr = null;
+            this.right_ptr = null;
+        }
+    }
+*/
+
+/**
+  * @param {TreeNode} root Root of the input tree.
+  * @return {TreeNode} Root of the output tree.
+  */
+
+var flipUpsideDown = function (root) {
+    if (!root) { return root; }
+
+    function helper(currentNode, prevNode) {
+        if (!currentNode) {
+            return prevNode;
+        }
+
+        prevNode.left_ptr = null;
+        const newRootNode = helper(currentNode.left_ptr, currentNode);
+
+        currentNode.right_ptr = prevNode;
+        currentNode.left_ptr = prevNode.right_ptr;
+        prevNode.right_ptr = null;
+
+        return newRootNode;
+    }
+
+    const rootNode = helper(root.left_ptr, root);
+    return rootNode;
+};

leetcode practice/trees and graphs/countUnivalSubtrees.js

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+/**
+ * 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 {TreeNode} root
+ * @return {number}
+ */
+
+function countUnivalSubtrees(root) {
+    let counter = 0;
+
+    if (!root) { return 0; }
+
+    function helper(node) {
+        if (!node.left && !node.right) {
+            counter++;
+            return true;
+        }
+
+        const leftValid = !node.left || (helper(node.left) && node.left.val === node.val);
+        const rightValid = !node.right || (helper(node.right) && node.right.val === node.val);
+
+        if (leftValid && rightValid) {
+            counter++;
+            return true;
+        }
+
+        return false;
+    }
+
+    helper(root);
+
+    return counter;
+}