https://leetcode.com/problems/inorder-successor-in-bst/

inorderSuccessor.js

/**
 * // Definition for a Node.
 * function Node(val) {
 *    this.val = val;
 *    this.left = null;
 *    this.right = null;
 *    this.parent = null;
 * };
 */

/**
 * @param {Node} node
 * @return {Node}
 */
var inorderSuccessor = function(node) {
    let [parent, child] = [node, node.right];
    
    if(child){ // find the left-most child of the right child
        while(child){
            parent = child;
            child = child.left;
        }
        return parent;
    };
    
    [child, parent] = [node, node.parent];
    while(parent){ // find the first right-parent
        if(child === parent.left) return parent;
        child = parent;
        parent = parent.parent;
    }
    
    return null;
};