Even i got it passed, the testcase still looks a bit strange to me

When the input is null, technically speaking, it is not even a tree, shouldn't the result be false?

```
Input: {}
Output: false
Expected: true
```

Here is the accepted code in javascript

```
/**
* Memo: Kinda werid to see the result is true when root is null.
* Runtime: 161ms
* Rank: S
*/
var isBalanced = function(root) {
if (!root) {
return true; // doesn't look so right here.
}
var left = isBalanced(root.left);
var right = isBalanced(root.right);
if (left && right) {
var dLeft = getDepth(root.left);
var dRight = getDepth(root.right);
return Math.abs(dLeft - dRight) <= 1;
} else {
return false;
}
};
var getDepth = function(root) {
if (!root) {
return 0;
}
return Math.max(getDepth(root.left), getDepth(root.right)) + 1;
};
```