問題描述

Given a binary tree, you need to compute the length of the diameter of the tree. The diameter of a binary tree is the length of the longest path between any two nodes in a tree. This path may or may not pass through the root.

思路

• 建一個(gè)全局變量count，存當(dāng)前最大值，并且不受到recursive的影響
• 在大方法里，傳root，返回count
• 用一個(gè)recursive方法求樹的高度，如果node為None，返回0；每傳進(jìn)一個(gè)node, 先拿到左右兩邊的subtree分別的高left和right，count = max(count, left + right)；返回max(left, right)+1

class Solution:
    count = 0
    def diameterOfBinaryTree(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        self.getH(root)
        return self.count

    def getH(self, node):

        if not node:
            return 0
        left = self.getH(node.left)
        right = self.getH(node.right)
        self.count = max(self.count, left + right)     #update count
        return max(left, right)+1