My Java recursive solution (1 ms)

[nagorishaily]

class Solution {
    public int maxDepth(TreeNode root) {
        int leftDepth = 0 ;
        int rightDepth = 0; 
        int maxDepth = 0;
        if (root == null) return 0;
        else {
            leftDepth += 1 + maxDepth(root.left);
            rightDepth += 1 + maxDepth(root.right);
            maxDepth = Math.max(leftDepth,rightDepth);
            return maxDepth;
        }
        
    }
}