https://leetcode.com/problems/maximum-depth-of-binary-tree/description/

MaxDepthOfBinaryTree.java

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;
 }
}

public class MaxDepthOfBinaryTree {
    int maxDepth = 0;
    int currentDepth = 0;
    public int maxDepth(TreeNode root) {
        if(root!=null){
            currentDepth++;
            maxDepth = currentDepth>maxDepth?currentDepth:maxDepth;
            if(root.left!=null)
                maxDepth(root.left);
            if(root.right!=null)
                maxDepth(root.right);
            currentDepth--;
        }
        return maxDepth;
    }
}