Maximum Depth of Binary Tree Solutions in Java

// Source : https://oj.leetcode.com/problems/maximum-depth-of-binary-tree/
// Inspired by : http://www.jiuzhang.com/solutions/maximum-depth-of-binary-tree/
// Author : Lei Cao
// Date   : 2015-10-03




package maximumDepthOfBinaryTree;

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class maximumDepthOfBinaryTree {
    public int maxDepth(TreeNode root) {
        if (root == null) {
            return 0;
        }
        return theDepth(root);
    }

    private int theDepth(TreeNode root) {
        int leftDepth = 1;
        int rightDepth = 1;

        if (root.left != null) {
           leftDepth = theDepth(root.left) + 1;
        }
        if (root.right != null) {
            rightDepth = theDepth(root.right) + 1;
        }
        return Math.max(leftDepth, rightDepth);
    }
}

/// Source : https://leetcode.com/problems/maximum-depth-of-binary-tree/description/
/// Author : liuyubobobo
/// Time   : 2017-11-17
///
/// Recursive
/// Time Complexity: O(n), where n is the nodes' number in the tree
/// Space Complexity: O(h), where h is the height of the tree
class Solution1 {

    // Definition for a binary tree node.
    public class TreeNode {
        int val;
        TreeNode left;
        TreeNode right;
        TreeNode(int x) { val = x; }
    }

    public int maxDepth(TreeNode root) {

        if(root == null)
            return 0;

        return 1 + Math.max(maxDepth(root.left), maxDepth(root.right));
    }
}

/// Source : https://leetcode.com/problems/maximum-depth-of-binary-tree/description/
/// Author : liuyubobobo
/// Time   : 2017-11-17
import java.util.Stack;
import javafx.util.Pair;

/// Non-recursive
/// Time Complexity: O(n), where n is the nodes' number in the tree
/// Space Complexity: O(h), where h is the height of the tree
class Solution2 {

    // Definition for a binary tree node.
    public class TreeNode {
        int val;
        TreeNode left;
        TreeNode right;
        TreeNode(int x) { val = x; }
    }

    public int maxDepth(TreeNode root) {

        if(root == null)
            return 0;

        Stack<Pair<TreeNode, Integer>> s = new Stack<Pair<TreeNode, Integer>>();
        s.add(new Pair(root, 1));
        int res = 0;
        while(!s.empty()){
            TreeNode curNode = s.peek().getKey();
            int depth = s.peek().getValue();
            s.pop();

            if(curNode.left == null && curNode.right == null)
                res = Math.max(res, depth);
            else{
                if(curNode.left != null)
                    s.push(new Pair(curNode.left, depth + 1));
                if(curNode.right != null)
                    s.push(new Pair(curNode.right, depth + 1));
            }
        }

        return res;
    }
}