LeetCode#110 Balanced Binary Tree

Problem Definition:

Given a binary tree, determine if it is height-balanced.

For this problem, a height-balanced binary tree is defined as a

binary tree in which the depth of the two subtrees of every node never differ by more than 1.

这里面临的问题是，既要记录子树的高度，又要记录子树是否平衡。

Solution 1: 只记录子树是否平衡，每次都重新计算高度，太耗时。

Solution 2: 用一个tuple来记录高度和平衡。

    # @param {TreeNode} root
    # @return {boolean}
    def isBalanced(root):
        b,h=func(root)
        return b

    def func(root):
        if root==None:
            return (True,0)
        lb,lh=func(root.left)
        rb,rh=func(root.right)
        return (abs(lh-rh)<=1 and lb and rb, max(lh,rh)+1)
    

Solution 3: 把这两个变量结合在一起，用一个负数表示不平衡时的高度。

    def isBalanced(self, root):
        return self.height(root)!=-1

    def height(self,root):
        if root==None:
            return 0
        hl=self.height(root.left)
        hr=self.height(root.right)
        if hl==-1 or hr==-1 or abs(hl-hr)>1:
            return -1
        return max(hl,hr)+1