Sum of Left Leaves

Problem statement

Given the root of a binary tree, return the sum of all left leaves.

Example 1:

Input: root = [3,9,20,null,null,15,7]Output: 24Explanation: There are two left leaves in the binary tree, with values 9 and 15 respectively.

Example 2:

Input: root = [1]Output: 0

Constraints:

• The number of nodes in the tree is in the range [1, 1000].
• -1000 <= Node.val <= 1000

My solution

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number}
 */
var sumOfLeftLeaves = function(root) {
    let sum = 0;
    
    const stack = [root];
    
    while (stack.length > 0) {
        const node = stack.pop();
        
        // console.log(node)
        
        if (!node) {
            return sum;
        }
        
        if (node.left) {
            stack.push(node.left)
            
            if (!node.left.left && !node.left.right) {
                sum += node.left.val;
            }
        }
        
        if (node.right) {
            stack.push(node.right)
        }
        
    }
    
    // console.log("sum", sum)
    return sum
};