110. Balanced Binary Tree

public boolean isBalanced(TreeNode root) {
if(root == null ) {
return true;
}
return depth(root) >= 0;
}

private int depth(TreeNode root) {
if(root == null) {
return 0;
}
int left = depth(root.left);
int right = depth(root.right);
if(left == -1 || right == -1) {
return -1;
}
if(Math.abs(left - right) > 1) {
return -1;
}
return Math.max(left, right) + 1;
}