数据结构里面讲过：用递归，深度优先遍历或广度优先遍历都行
/* // Definition for a Node. class Node { public int val; public List<Node> children; public Node() {} public Node(int _val) { val = _val; } public Node(int _val, List<Node> _children) { val = _val; children = _children; } }; */
class Solution { public int maxDepth(Node root) { if(root == null) return 0; int max = 0; for(Node node : root.children){ //深度优先
int count = maxDepth(node); if (max < count) max = count; } return max + 1; }