Find the Kth largest element in a stream using a BST.

FindKthLargest-BST.js

function Node(val) {
    this.val = val
    this.rightCount = 1
    this.left = null
    this.right = null
}

/**
 * @param {number} k
 * @param {number[]} nums
 */
var KthLargest = function(k, nums) {
    this.k = k
    this.root = new Node(nums[0])
    let i = 1
    while(i < nums.length) {
        this.addNode(nums[i], this.root)
        i++
    }
};

KthLargest.prototype.addNode = function (val, current) {
    if (val < current.val) {
        if (current.left) {
            this.addNode(val, current.left)   
        } else {
            current.left = new Node(val)
        }
    } else {
        current.rightCount += 1
        if (current.right) {
            this.addNode(val, current.right)
        } else {
            current.right = new Node(val)
        }
    }
}

KthLargest.prototype.find = function (current, k) {
    if (current) {
        if (current.rightCount === k) {
            return current
        } else if (current.rightCount > k) {
            return this.find(current.right, k)
        } else {
            return this.find(current.left, k - current.rightCount)
        }
    } else {
        return null
    }
}

/** 
 * @param {number} val
 * @return {number}
 */
KthLargest.prototype.add = function(val) {
    this.addNode(val, this.root)
    var result = this.find(this.root, this.k)
    return result ? result.val : null
};

/** 
 * Your KthLargest object will be instantiated and called as such:
 * var obj = Object.create(KthLargest).createNew(k, nums)
 * var param_1 = obj.add(val)
 */