104. Maximum Depth of Binary Tree

递归时，每层路径加一

public int maxDepth(TreeNode root) {
    if(root == null)  return 0;
    return 1 + Math.max(maxDepth(root.left), maxDepth(root.right));
}

    3
   / \ +1
  9  20
    /  \ +1
   15   7
         \ + 1
         null

求二叉树节点最远距离: 重点是这个公式max_dis = max(l_depth + r_depth, max(l_dis, r_dis))，是说最远距离=max(左边最大深度+右边最大深度,max(左边最大距离,右边最大距离))

def maxDis(self, root):
    return self.dfs(root)

def dfs(self, root):
    if not root: 
        return [0, -1]                                                             
    l_depth, l_dis = self.dfs(root.left)
    r_depth, r_dis = self.dfs(root.right)
    max_depth = max(l_depth, r_depth) + 1
    max_dis = max(l_depth + r_depth, max(l_dis, r_dis))
    return [max_depth, max_dis]

测试：

[3,9,20,null,null,15,7]
[1,2,3,4,5,6,7,8,null,null,null,null,9]
[1,2,null,3,4,5,null,null,6]

result: 
[3,3]
[4,6]
[4,4]