tree_traversal.js

function levelOrder (node, visit) {
  'use strict'

  let queue = [node]
  while (queue.length > 0) {
    node = queue.pop()
    if (!node) {
      continue
    }

    visit(node)
    queue.push(node.left, node.right)
  }
}

function preOrder (node, visit) {
  'use strict'
  if (!node) {
    return
  }
  preOrder(node.left, visit)
  preOrder(node.right, visit)
  visit(node)
}

function inOrder (node, visit) {
  'use strict'
  if (!node) {
    return
  }
  inOrder(node.left, visit)
  visit(node)
  inOrder(node.right, visit)
}

function postOrder (node, visit) {
  'use strict'
  if (!node) {
    return
  }
  postOrder(node.left, visit)
  postOrder(node.right, visit)
  visit(node)
}

var node = {
  value: 10,
  left: {
    value: 20,
    right: {
      value: 30,
      left: {
        value: 40
      },
      right: {
        value: 50,
      }
    }
  }
}

console.log('# levelOrder #')
levelOrder(node, function (n) {
  console.log(n.value)
})

console.log('# postOrder #')
preOrder(node, function (n) {
  console.log(n.value)
})

console.log('# inOrder #')
inOrder(node, function (n) {
  console.log(n.value)
})

console.log('# postOrder #')
postOrder(node, function (n) {
  console.log(n.value)
})

# levelOrder #
10
20
30
50
40
# postOrder #
40
50
30
20
10
# inOrder #
20
40
30
50
10
# postOrder #
40
50
30
20
10