563. slope of binary tree

subject ：
Answer key ：
Ideas ： Recursive method , Mainly determine the end condition of recursion
Code ：

var findTilt = function (root) {
    let start = 0
    mutation(root)
    return start

    function mutation(root) {
        if (root == null) {
            return 0
        }
        let left = mutation(root.left)
        let right = mutation(root.right)

        start += Math.abs(left - right)
        return left + right + root.val
    }
};

ps（ On the third day )