Sum Root to Leaf Numbers.md

DFS T.C. O(N)

/**
 * 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) {
    let sum = 0;
    
    function dfs(node, num) {
        num += node.val;
        if(node.left === null && node.right === null) {
            sum += +num;
        }
        if(node.left) dfs(node.left, num);
        if(node.right) dfs(node.right, num);
    }
    
    dfs(root, '');
    return sum;
};