Given a binary tree, find its minimum depth.

The minimum depth is the number of nodes along the shortest path from the root node down to the nearest leaf node.

 //深度優先  維護最小深度
    public int minDepth(TreeNode root) {

        if(root ==null){//遞迴結束
            return 0;
        }   
        int left = minDepth(root.left);
        int right = minDepth(root.right);
        if(left!=0 && right!=0){
            return Math.min(left, right)+1;//當前節點最小深度
        }
        if(left ==0){
            return right+1;
        }

        return left+1;
    }