Leetcode 0104. Maximum Depth of Binary Tree

Given the `root` of a binary tree, return its maximum depth. A binary tree's maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

Easy

Description

Given the root of a binary tree, return its maximum depth.

A binary tree’s maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

Example 1:

Input: root = [3,9,20,null,null,15,7]
Output: 3

Example 2:

Input: root = [1,null,2]
Output: 2

Constraints:

• The number of nodes in the tree is in the range [0, 10^4].
• -100 <= Node.val <= 100

Solution

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    /**
     * Iteration
     * Time Complexity: BigO(N)
     * Space Complexity: BigO(N)
     */
    public int maxDepth(TreeNode root) {
        if (root == null) return 0;

        int max = 0;
        Stack<TreeNode> stack = new Stack<>();
        Stack<Integer> depth = new Stack<>();
        stack.push(root);
        depth.push(1);

        while(!stack.empty()) {
            TreeNode node = stack.pop();
            int currDepth = depth.pop();
            max = Math.max(currDepth, max);
            if (node.left != null) {
                stack.push(node.left);
                depth.push(currDepth + 1);
            }
            if (node.right != null) {
                stack.push(node.right);
                depth.push(currDepth + 1);
            }
        }
        return max;
    }
}