LeetCode 226 - Invert Binary Tree

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {TreeNode}
 */
var invertTree = function(root) {
    // Recursion must have a base condition.
    if(root == null) {
        return root;
    }
    
    // A simple swap of let & right nodes of a root.
    let tmp = root.right;
    root.right = root.left;
    root.left = tmp;
    
    // Recurse on both sides in Preorder Traversal.
    invertTree(root.left);
    invertTree(root.right);
    
    return root;
};

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {TreeNode}
 */
var invertTree = function(root) {
    if(root == null) {
        return root;
    }
    
    let stack = [root], node, tmp;
    
    // Use stack as this problem is a variant of Depth First Traversal
    while(stack.length) {
        node = stack.pop();
        
        if(node == null) {
            continue;
        }
        
        // Simple swap of left & right node
        tmp = node.right;
        node.right = node.left;
        node.left = tmp;
        
        stack.push(node.left);
        stack.push(node.right);
    }
    
    return root;
};