依次遍历num中每个元素 删除nums[i+1]<nums[i]的元素i 
注意边界问题 （1）头部是0 忽略（2）结束时k>0 从后向前删除 直到k=0

    int k;
    String nums;
    Scanner in = new Scanner(System.in);
    nums = in.next();
    k = in.nextInt();
    if(nums==null||nums.length()==0){
       System.out.print("0");
       return;
    } 
    StringBuilder ans = new StringBuilder();
    for(int i=0; i<nums.length(); i++){
        while(ans.length()>0&&nums.charAt(i)-'0'<ans.charAt(ans.length()-1)-'0'&&k>0){//删除num[i]
            ans.deleteCharAt(ans.length()-1);
            k--;
        }
        if(ans.length()==0&&nums.charAt(i)=='0') continue;//头部是0 忽略
        ans.append(nums.charAt(i));
    }
    while(k>0&&ans.length()>0){//k>0
        ans.deleteCharAt(ans.length()-1);
        k--;
    } 
    if(ans.length()==0){
       System.out.print("0");
    }else
        System.out.print(ans.toString());