leetcode104. maximum depth of binary tree

public int maxDep = 0;
public int (TreeNode root) {
    if (root != null) preorderWalk(root, 1);
    return maxDep;
}
public void preorderWalk(TreeNode root, int level) {
    if (root == null) return;
    if (root.left == null && root.right == null) if (level > maxDep) maxDep = level;
    preorderWalk(root.left, level+1);
    preorderWalk(root.right, level+1);
}