unlockDescendants(index) {
        const stack = [];
        stack.push(index);
        while(stack.length) {
            const node = stack.pop();
            const children = this.childHash[node];
            for(let i = 0; i < (children && children.length); i++) {
                delete this.treeHash[children[i]];
                stack.push(children[i]);
            }
        }
    }
Helper method to unlock descendants
Time O(n) | Space O(n)
@param {number} index