```
var findTilt = function(root) {
let tilt = 0;
function postOrder(root) {
if (root === null) {
return 0;
}
let left = postOrder(root.left);
let right = postOrder(root.right);
tilt += Math.abs(left - right);
return root.val + left + right;
}
postOrder(root);
return tilt;
};
```