public int minDepth(TreeNode root) {
        // Start typing your Java solution below
        // DO NOT write main() function

        if (root == null) {
            return 0;
        } else if (root.left == null && root.right == null) {
            return 1;
        } else if (root.left == null) {
            return 1 + minDepth(root.right);
        } else if (root.right == null) {
            return 1 + minDepth(root.left);
        } else {
            return 1 + Math.min(minDepth(root.left), minDepth(root.right));
        }
    }