[LeetCode] 559. Maximum Depth of N-ary Tree

Given a n-ary tree, find its maximum depth.

The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

For example, given a 3-ary tree:

 

We should return its max depth, which is 3.

Note:

1. The depth of the tree is at most 1000.
2. The total number of nodes is at most 5000.

 

Solution 1. Recursion

 

/*
// Definition for a Node.
class Node {
    public int val;
    public List<Node> children;

    public Node() {}

    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 child : root.children) {
            max = Math.max(maxDepth(child), max);
        }
        return max + 1;
    }
}

Solution 2. BFS(tracking the number of nodes that need to be visited at each level)

class Solution {
    public int maxDepth(Node root) {
        if(root == null) {
            return 0;
        }
        int level = 0;
        int currLevelCount = 1;
        int nextLevelCount = 0;
        Queue<Node> queue = new LinkedList<>();
        queue.add(root);
        while(!queue.isEmpty()) {
            Node currNode = queue.poll();
            currLevelCount--;
            for(Node child : currNode.children) {
                queue.add(child);
                nextLevelCount++;
            }            
            if(currLevelCount == 0) {
                level++;
                currLevelCount = nextLevelCount;
                nextLevelCount = 0;
            }
        }
        return level;
    }
}

Follow up:  Can you implement a BFS to a certain depth?