A binary tree is a tree where each node may only have up to two children. These children are stored on the leftand right properties of each node.

When traversing a binary tree, we have three common traversal algorithms: in order, pre-order, and post-order. In this lesson, we write each of these algorithms and explore their differences.

// Binary Trees and Tree Traversal

// Binary trees are trees whose nodes can only have up to two children

function createBinaryNode(key) {
return {
key,
left: null,
right: null,
addLeft(leftKey) {
const newLeft = createBinaryNode(leftKey)
this.left = newLeft
return newLeft
},
addRight(rightKey) {
const newRight = createBinaryNode(rightKey)
this.right = newRight
return newRight
}
}
} const TRAVERSALS = {
/**
* Javascript Call stack is Last in, First Out,
* So it keep calling
* TRAVERSALS.IN_ORDER(node.left, visitFn)
* Until it reach the bottom left node 'h' (b- d- h)
* h - visitFn get called
* h - TRAVERSALS.IN_ORDER(node.right, visitFn) get called
*
* d - visitFn get called
* d - left
* d - right
*
* b - visitFn
* b - left
* b - right
*/
IN_ORDER: (node, visitFn) => {
if (node !== null) {
console.log('left', node.left && node.left.key)
TRAVERSALS.IN_ORDER(node.left, visitFn)
console.log('call', node.key)
visitFn(node)
console.log('right', node.right && node.right.key)
TRAVERSALS.IN_ORDER(node.right, visitFn)
}
},
PRE_ORDER: (node, visitFn) => {
if (node !== null) {
visitFn(node)
TRAVERSALS.PRE_ORDER(node.left, visitFn)
TRAVERSALS.PRE_ORDER(node.right, visitFn)
}
},
POST_ORDER: (node, visitFn) => {
if (node !== null) {
TRAVERSALS.POST_ORDER(node.left, visitFn)
TRAVERSALS.POST_ORDER(node.right, visitFn)
visitFn(node)
}
}
} function createBinaryTree(rootKey) {
const root = createBinaryNode(rootKey) return {
root,
print(traversalType = 'IN_ORDER') {
let result = '' const visit = node => {
result += result.length === 0 ? node.key : ` => ${node.key}`
} TRAVERSALS[traversalType](this.root, visit) return result
}
}
} const tree = createBinaryTree('a')
const b = tree.root.addLeft('b')
const c = tree.root.addRight('c')
const d = b.addLeft('d')
const e = b.addRight('e')
const f = c.addLeft('f')
const g = c.addRight('g')
const h = d.addLeft('h')
const i = d.addRight('i') console.log('IN_ORDER: ', tree.print())// IN_ORDER: h => d => i => b => e => a => f => c => g
//console.log('PRE_ORDER: ', tree.print('PRE_ORDER')) // PRE_ORDER: a => b => d => h => i => e => c => f => g
  //console.log('POST_ORDER: ', tree.print('POST_ORDER')) // POST_ORDER: h => i => d => e => b => f => g => c => a
  exports.createBinaryNode = createBinaryNode
exports.createBinaryTree = createBinaryTree

Time complexity: O(n),

Space Complexity O(h) for average cases; h = logN -- this is because we need to stack all the function calls. worse cases: O(n)

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