Balanced Binary Tree

def isBalanced(root):
    def dfs(node):
        if not node:
            return [True, 0]
            
        left = dfs(node.left)
        right = dfs(node.right)
        
        balanced = (left[0] and right[0] and 
                    abs(left[1] - right[1]) <= 1)
                    
        return [balanced, 1 + max(left[1], right[1])]
        
    return dfs(root)[0]