110. 平衡二叉树

方法一：自底向上

class Solution(object):
    # 法一：自底向上
    def isBalanced(self, root):
        """
        :type root: TreeNode
        :rtype: bool
        """
        return self.helper(root) >= 0

    def helper(self, root):
        if not root:
            return 0
        left = self.helper(root.left)
        right = self.helper(root.right)
        if left == -1 or right == -1 or abs(left - right) > 1:
            return -1
        return max(left, right) + 1

方法二：自顶向下

class Solution(object):
    # 法二：自顶向下
    def isBalanced(self, root):
        """
        :type root: TreeNode
        :rtype: bool
        """
        if not root:
            return True

        # 左、右子树深度
        heightLeft = self.treeDepth(root.left)
        heightRight = self.treeDepth(root.right)
        # 判断左、右子树是否平衡
        resultLeft = self.isBalanced(root.left)
        resultRight = self.isBalanced(root.right)

        # 左子树和右子树都是AVL，并且高度相差不大于1，返回真
        if resultLeft and resultRight and abs(heightLeft - heightRight) <= 1:
            return True
        else:
            return False

    # 求二叉树的深度
    def treeDepth(self, root):
        if not root:
            return 0
        depthLeft = self.treeDepth(root.left)
        depthRight = self.treeDepth(root.right)
        return depthLeft + 1 if depthLeft > depthRight else depthRight + 1