#129. Sum Root to Leaf Numbers

/**
 * 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 sumNumbers = function (root) {
    if (!root) return

    let sum = 0

    function dfs(node, currentSum) {

        if (!node) return

        currentSum = (currentSum * 10) + node.val 

        if (!node.right && !node.left) {
            sum += currentSum 
            return
        }

        dfs(node.right, currentSum)
        dfs(node.left, currentSum)
    }

    dfs(root, 0)
    return sum
};