1522. Diameter of N-Ary Tree

Given a root of an N-ary tree, you need to compute the length of the diameter of the tree.

The diameter of an N-ary tree is the length of the longest path between any two nodes in the tree. This path may or may not pass through the root.

(Nary-Tree input serialization is represented in their level order traversal, each group of children is separated by the null value.)

 

Example 1:

Input: root = [1,null,3,2,4,null,5,6]
Output: 3
Explanation: Diameter is shown in red color.

Example 2:

Input: root = [1,null,2,null,3,4,null,5,null,6]
Output: 4

Example 3:

Input: root = [1,null,2,3,4,5,null,null,6,7,null,8,null,9,10,null,null,11,null,12,null,13,null,null,14]
Output: 7

class Solution {
    int max = 0;
    public int diameter(Node root) {
        max = 0;
        getDiameter(root);
        return max;
    }

    private int getDiameter(Node node) {
        if(node == null) {
            return 0;
        }
        List<Node> children = node.children;
        int maxOne = 0; // max from all children
        int maxTwo = 0; // second max from all children
        for (Node child :children) {
            int res = getDiameter(child);
            if (res > maxOne) {
                maxTwo = maxOne;
                maxOne = res;
            } else if (res > maxTwo) {
                maxTwo = res;
            }
        }
        max = Math.max(max, maxOne + maxTwo);
        return maxOne + 1;
    }
}