Added helpers file for ch4 solutions, missed in previous commits.

Author: diadical

src/chapter4/helpers.js

@@ -0,0 +1,103 @@
+
+class TreeNode {
+  constructor(value) {
+    this.val = value;
+    this.parent = this.left = this.right = null;
+  }
+}
+
+export class Tree {
+  constructor() {
+    this.root = null;
+  }
+
+  add(value) {
+    let node = new TreeNode(value);
+    if (!this.root) {
+      this.root = node;
+    }
+    else {
+      let n = this.root,
+        branch;
+      while (n) {
+        branch = value < n.val ? 'left' : 'right';
+        if (!n[branch]) {
+          break;
+        }
+        n = n[branch];
+      }
+      node.parent = n;
+      n[branch] = node;
+    }
+  }
+}
+
+class LinkedListNode {
+  constructor(value, next) {
+    this.val = value;
+    this.next = next || null;
+  }
+}
+
+export class LinkedList {
+  constructor() {
+    this.head = this.tail = null;
+  }
+
+  prepend(value) {
+    if (!this.head) {
+      this.head = this.tail = new LinkedListNode(value);
+    }
+    else {
+      this.head = new LinkedListNode(value, this.head);
+    }
+  }
+
+  append(value) {
+    if (!this.head) {
+      this.head = this.tail = new LinkedListNode(value);
+    }
+    else {
+      this.tail = this.tail.next = new LinkedListNode(value);
+    }
+  }
+
+  toArray() {
+    let arr = [],
+      node = this.head;
+    while (node) {
+      arr.push(node.val);
+      node = node.next;
+    }
+    return arr;
+  }
+}
+
+function findDepth(cache, node, depth) {
+  if (!node) {
+    if (depth < cache.min) {
+      cache.min = depth;
+    }
+    if (depth > cache.max) {
+      cache.max = depth;
+    }
+  } else {
+    findDepth(cache, node.left, depth + 1);
+    findDepth(cache, node.right, depth + 1);
+  }
+}
+
+export function isBalanced(tree) {
+  if (!tree || !tree.root) {
+    return true;
+  }
+
+  let node = tree.root,
+    cache = {
+      min: Number.MAX_SAFE_INTEGER,
+      max: Number.MIN_SAFE_INTEGER
+    };
+
+  findDepth(cache, node, 0);
+  return cache.max - cache.min <= 1;
+}