Leetcode - Maximum Depth of Binary Tree

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Maximum Depth of Binary Tree

Given a binary 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.

Solution:
1. Recursive:

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public int maxDepth(TreeNode root) {
        if(root == null)    return 0;
        
        return getDepth(root, 1);
    }
    
    public int getDepth(TreeNode node, int depth) {
        int left = depth, right = depth;
        if(node.left != null) left = getDepth(node.left, depth + 1);
        if(node.right != null) right = getDepth(node.right, depth + 1);
        
        return left > right ? left : right;
    }
}

2. Non-recursive:

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public int maxDepth(TreeNode root) {
        if(root == null)    return 0;
        
        // Non-recursive, use level order triversal
        ArrayList<TreeNode> q = new ArrayList<TreeNode>();
        q.add(root);
        int depth = 0;
        
        while(!q.isEmpty()) {
            ArrayList<TreeNode> next = new ArrayList<TreeNode>();
            for(TreeNode node : q) {
                if(node.left != null)   next.add(node.left);
                if(node.right != null)  next.add(node.right);
            }
            q = new ArrayList<TreeNode>(next);
            depth++;
        }
        
        return depth;
    }
}