var preorderTraversal = function(root, res = []) { if (!root) return []; res.push(root.val); if(root.left) preorderTraversal(root.left, res); if(root.right) preorderTraversal(root.right, res); return res; };

var preorderTraversal = function(root) { if(!root) return []; //We need to put a root in the stack first, because Pre-order traverse from root => left => right let stack = [root]; let arr = []; while(stack.length) { let curr = stack.pop(); arr.push(curr.val); // we want to push right subtree is because the left subtree will be visit first then. if(curr.right) { stack.push(curr.right); } if(curr.left) { stack.push(curr.left); } } return arr; };