missingNumber task - binary tree implementation.js

function insert (node, x) {
    if (x < node.value) {
        if (node.left) {
            if (x < node.left.value) {
                insert(node.left, x)
            } else {
                node.left = {value: x, left: node.left}
            }
        } else {
            node.left = {value: x}
        }
    } else {
        if (node.right) {
            if (x > node.right.value) {
                insert(node.right, x)
            } else {
                node.right = {value: x, right: node.right}
            }
        } else {
            node.right = {value: x}
        }
    }
}

function check (node) {
    if (node.left) {
        if (node.value - node.left.value > 1) {
            return node.value - 1 
        }
    }
    if (node.right) {
        if (node.right.value - node.value > 1) {
            return node.value + 1
        }
    }
    return node.left && check(node.left) || node.right && check(node.right)
}

function missingNumber(a) {
    if (a.length === 1) {
        return a[0] === 0 ? 1 : 0 
    }
    var tree = {value: a[0]}
    for (var i = 1; i < a.length; i++) {
        insert(tree, a[i])
    }
    var missed = check(tree)
    var node = tree
    while (node.left) node = node.left
    return missed === undefined
        ? node.value === 0
            ? a.length
            : 0
        : missed
}