【Leetcode】543. Diameter of Binary Tree解题报告

版权声明：本文为博主原创文章，未经博主允许不得转载。 https://blog.csdn.net/dpengwang/article/details/88073791

求二叉树最大直径

方法1 递归法

class Solution1(object):
    def diameterOfBinaryTree(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        self.res =  0
        self.DFS(root)
        return self.res
    def DFS(self,root):
        if root == None:
            return 0
        l = self.DFS(root.left)
        r = self.DFS(root.right)
        self.res = max(self.res, l + r )
        return max(l, r) +1

方法2 后续非递归

后续非递归遍历加dict保存每个节点的最大深度
因为每个节点只有遍历完其左子树和右子树后才能获得其最大深度，所以得postorder

class Solution2(object):
    def diameterOfBinaryTree(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        from collections import  defaultdict
        dictionary = defaultdict(int)
        stack, pNode, prev,res = [], root, None,0
        while(len(stack) or pNode):
            if pNode:
                stack.append(pNode)
                pNode = pNode.left
            else:
                pNode = stack[-1]
                if pNode.right == None or pNode.right == prev:
                    curr_path = dictionary[pNode.left] + dictionary[pNode.right]
                    res = max(res, curr_path)
                    dictionary[pNode] = max(dictionary[pNode.left], dictionary[pNode.right]) + 1
                    prev = pNode
                    pNode = None
                    stack.pop()
                else:
                    pNode = pNode.right
        return res