kth-smallest-in-bst.js

// O(n) space complexity solution
var kthSmallest = function(root, k) {
    var stack  = [];
    var count = 0;
    var node = root;
    
    while (node) {
        stack.push(node);
        node = node.left;
    }
    
    while (stack.length > 0) {
        node = stack.pop();
        count++;
        
        if (count === k) {
            return node.val;
        }
        
        node = node.right;
        
        while (node) {
            stack.push(node);
            node = node.left;
        }
    }
    
    return null;
};