4.5 is a binary search tree.js - works for all cases

Author: amejiarosario

src/trees-and-graphs/is-binary-search-tree.js

@@ -3,23 +3,33 @@
  * @param node
  * @returns {boolean}
  */
-function isBinarySearchTree(node) {
+function isBinarySearchTree(node, min, max) {
   if(!node) return true;
 
   const parent = node.data;
-  let isBST = true;
+  let isBST = true, left, right;
 
   if(node.left) {
-    const left = node.left.data;
-    isBST = isBST && left < parent;
+    left = node.left.data;
+    isBST = isBST && left <= parent;
+
+    if(min) {
+      isBST = isBST && left > min;
+    }
   }
 
   if(node.right) {
-    const right = node.right.data;
+    right = node.right.data;
     isBST = isBST && right > parent;
+
+    if(max) {
+      isBST = isBST && right < max;
+    }
   }
 
-  return isBST && isBinarySearchTree(node.left) && isBinarySearchTree(node.right);
+  return isBST &&
+    isBinarySearchTree(node.left, null, parent) &&
+    isBinarySearchTree(node.right, parent, null);
 }
 
 module.exports = isBinarySearchTree;

src/trees-and-graphs/is-binary-search-tree.spec.js

@@ -27,19 +27,23 @@ describe('Tree: is binary tree search', function () {
 
   it('should work with valid data', function () {
     let
-      a = new Node('a'),
-      b = new Node('b'),
-      c = new Node('c'),
-      d = new Node('d'),
-      e = new Node('e'),
-      f = new Node('f'),
-      g = new Node('g');
+      a = new Node(1),
+      b = new Node(2),
+      c = new Node(3),
+      d = new Node(4),
+      e = new Node(5),
+      f = new Node(6),
+      g = new Node(7);
 
     d.left = c;
+    d.right = e;
+
     c.left = b;
 
-    d.right = e;
+    b.left = a;
+
     e.right = f;
+
     f.right = g;
 
     expect(isBinarySearchTree(d)).to.equal(true);
@@ -60,4 +64,19 @@ describe('Tree: is binary tree search', function () {
     expect(isBinarySearchTree(n20)).to.equal(false);
   });
 
+  it('should return false when a right child is less than root', function () {
+    let
+      n10 = new Node(10),
+      n20 = new Node(20),
+      n25 = new Node(25),
+      n30 = new Node(30);
+
+    n20.left = n10;
+    n20.right = n30;
+
+    n20.left = n25;
+
+    expect(isBinarySearchTree(n20)).to.equal(false);
+  });
+
 });