How much are smaller than I 2

binary_tree.js

class Node {
  constructor(value) {
    this.val = value;
    this.left = null;
    this.right = null;
    this.elemCount = 1
    this.leftCount = 0
  }
}

class Tree {
  constructor(root) {
    this.root = root;
  }

  insert(node) {
    let current = this.root;
    let count = 0;
    let prev;
    while(current !== null) {
      prev = current;
      if(node.val > current.val) {
        count += (current.elemCount + current.leftCount);
        current = current.right;
      } else if(node.val < current.val) {
        current.leftCount += 1;
        current = current.left;
      } else {
        current = null;
        prev.elemCount += 1
      }
    }
    if(prev.val > node.val) prev.left = node
    else if(prev.val < node.val) prev.right = node
    else return count + prev.leftCount;
    return count
  }
}

function smaller(arr) {
  const t = new Tree(new Node(arr[arr.length - 1]))
  const ans = [0]
  for(let i = arr.length-2; i > -1; i--) {
    ans.push(t.insert(new Node(arr[i])))
  }
  return ans.reverse();
}