Code in Javascript that performs a check for pre-order traversal

Question

Code in Javascript that performs a check for pre-order traversal

I’m interested in writing a preorder traversal code in Java for JavaScript. I’ve been practicing this question on geeksforgeeks:

Check if a given array can represent Preorder Traversal of Binary Search Tree

This is the algorithm they provided:

1) Create an empty stack.
2) Initialize root as INT_MIN.
3) Do following for every element pre[i]
     a) If pre[i] is smaller than current root, return false.
     b) Keep removing elements from stack while pre[i] is greater
        then stack top. Make the last removed item as new root (to
        be compared next).
        At this point, pre[i] is greater than the removed root
        (That is why if we see a smaller element in step a), we 
        return false)
     c) push pre[i] to stack (All elements in stack are in decreasing
        order)

I'm struggling to understand the above algorithm because I’m not sure what an empty stack is (possibly an empty array?), INT_MIN?

If an empty stack is an empty array, what does this statement mean?

Keep removing elements from stack while pre[i] is greater
than stack top. Make the last removed item the new root (to
be compared next).

In summary, I've tried to come up with an algorithm but need help making it more readable since I only know how to code in Javascript.

Could you assist me in improving the readability of the algorithm for the above code?

javascript

Answer 1

Answer №1

responding to your inquiries:

When referring to INT_MIN, the authors are indicating the smallest possible integer value, ensuring that any other number will be larger than INT_MIN. In JavaScript, you can use -Infinity for comparisons as it is considered smaller than any other numerical value. While mathematical operations cannot be performed on -Infinity, it is not necessary in this context.
A stack is a data structure similar to an array that allows elements to be added on top of each other and removed from the top, returning the most recently added element. To implement a stack in JavaScript, you can either 1. create a wrapper around an array with methods like .push, .pop, and
```
.peek</code, or 2. simply use an array and treat it as a stack. The latter option is more straightforward in this scenario.</li>
<li>Regarding the question "<code>If empty stack is represented by an empty array, then what does this statement mean?
```
", initially, the stack is empty. However, as you progress through the example, items are added to the stack, and there is a condition to check if the stack remains non-empty when reaching this particular statement.

.

const canRepresentBST = (pre) => {
  const stack = []
  let root = -Infinity

  for (let i = 0; i < pre.length; i++) {
    if (pre[i] < root) {
      return false
    }
    while (stack.length && stack[stack.length - 1] < pre[i]) {
      root = stack.pop()
    }
    stack.push(pre[i])
  }
  return true
}

const pre1 = [40,30,35,80,100]
console.log(canRepresentBST(pre1))
const pre2 = [40,30,35,20,80,100]
console.log(canRepresentBST(pre2))

The purpose of this algorithm is to determine whether a given array of numbers (pre) represents a valid preorder traversal of a binary search tree.

Answer 2

responding to your inquiries:

When referring to INT_MIN, the authors are indicating the smallest possible integer value, ensuring that any other number will be larger than INT_MIN. In JavaScript, you can use -Infinity for comparisons as it is considered smaller than any other numerical value. While mathematical operations cannot be performed on -Infinity, it is not necessary in this context.
A stack is a data structure similar to an array that allows elements to be added on top of each other and removed from the top, returning the most recently added element. To implement a stack in JavaScript, you can either 1. create a wrapper around an array with methods like .push, .pop, and
```
.peek</code, or 2. simply use an array and treat it as a stack. The latter option is more straightforward in this scenario.</li>
<li>Regarding the question "<code>If empty stack is represented by an empty array, then what does this statement mean?
```
", initially, the stack is empty. However, as you progress through the example, items are added to the stack, and there is a condition to check if the stack remains non-empty when reaching this particular statement.

.

const canRepresentBST = (pre) => {
  const stack = []
  let root = -Infinity

  for (let i = 0; i < pre.length; i++) {
    if (pre[i] < root) {
      return false
    }
    while (stack.length && stack[stack.length - 1] < pre[i]) {
      root = stack.pop()
    }
    stack.push(pre[i])
  }
  return true
}

const pre1 = [40,30,35,80,100]
console.log(canRepresentBST(pre1))
const pre2 = [40,30,35,20,80,100]
console.log(canRepresentBST(pre2))

The purpose of this algorithm is to determine whether a given array of numbers (pre) represents a valid preorder traversal of a binary search tree.

Answer 3

Answer №2

After reviewing your example, I successfully converted the java code to JavaScript.

const isBST = (preorder) => {
  let root = Number.MIN_SAFE_INTEGER
  let stack = []
  
  for (let i = 0; i < preorder.length; i++) {
    if (preorder[i] < root) {
      return false
    }
    
    while (stack.length > 0 && stack[stack.length-1] < preorder[i]) {
      root = stack.pop()
    }

    stack.push(preorder[i])
  }

  return true
}

let preorder1 = [40, 30, 35, 80, 100]
let preorder2 = [40, 30, 35, 20, 80, 100]

console.log(isBST(preorder1) ? 'true' : 'false')
console.log(isBST(preorder2) ? 'true' : 'false')

Answer 4

After reviewing your example, I successfully converted the java code to JavaScript.

const isBST = (preorder) => {
  let root = Number.MIN_SAFE_INTEGER
  let stack = []
  
  for (let i = 0; i < preorder.length; i++) {
    if (preorder[i] < root) {
      return false
    }
    
    while (stack.length > 0 && stack[stack.length-1] < preorder[i]) {
      root = stack.pop()
    }

    stack.push(preorder[i])
  }

  return true
}

let preorder1 = [40, 30, 35, 80, 100]
let preorder2 = [40, 30, 35, 20, 80, 100]

console.log(isBST(preorder1) ? 'true' : 'false')
console.log(isBST(preorder2) ? 'true' : 'false')

Code in Javascript that performs a check for pre-order traversal

Answer №1

Answer №2

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