LeetCode general Solution for BST In Order Traverse

In Order Traverse

There are a quite few leet code questions that is based on BST traverse.Here is a general template that could be used to solve similar problems.The idea is using a set to store the intermediate result.
The solution is in javascript.

LeetCode 94

 //Definition for a binary tree node.
var treeNode = function(val) {
     this.val = val;
     this.left = this.right = null;
 }

var inOrderTraverse = function (root) {
    
    let list = [];
    let stack = [];
    
    if (root == null) {
    
        return true;
    }
    
    while (root != null || stack.length > 0) {
        while (root != null) {
            stack.push(root);
            root = root.left;
        }    
        root = stack.pop();
        list.push(root.val);
        root = root.right;
    }
    
    return list;
}

LeetCode 230

var kthSmallest = function(root, k) {
    
    let stack = [];
    let list = [];
    
    if (root == null) {
        return true;
    }
    
    while (root != null || stack.length > 0) {
           
        while (root != null) {
            
            stack.push(root);
            root = root.left;
        }
        
        root = stack.pop();
        list.push(root.val);
        if (list.length == k) {
            return root.val;
        }
        root = root.right;
    }    
};

LeetCode 98

var isValidBST = function(root) {
    
    let stack = [];
    //let list = [];
    
    if (root === null) {
        return true;
    }
    
    let prev = null;
    
    while (root !== null || stack.length > 0) {
        
        while (root !== null) {
    
            stack.push(root);
            root = root.left;
        }
        
        root = stack.pop();
 
        if (prev != null && root.val <= prev.val) { 
  
            return false
        };
        
        prev = root;
        //list.push(root.val);
        root = root.right;
    }
        
    return true;  
};