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