Approaches

O(n) time. O(1) space. 10 lines.

go to each depth, incrementing a depth counter by 1. return max of it.

    public int maxDepth(TreeNode root, int depth){
        if(root == null){
            return depth;
        }
        int nextDepth = depth+1;
        return Math.max(maxDepth(root.left, nextDepth), maxDepth(root.right,nextDepth));
        
    }
    public int maxDepth(TreeNode root) {
        return maxDepth(root, 0);
    }

Code optimization: No need to create a separate function;

Optimized: 5 lines

Source: Java 100% Faster Very Simple - LeetCode Discuss

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