Remove K Digits

Given string A representative a positive integer which has N digits, remove any k digits of the number, the remaining digits are arranged according to the original order to become a new positive integer.

Find the smallest integer after remove k digits.

N <= 240 and k <= N,

Example

Given an integer A = "178542", k = 4

return a string "12"

Analysis:

When take current number n from string A, we need to compare the last digit we have in the new string, if n is greater than the last number in the new string, we do nothing. However, if n is less than the last digit in the new string, should we replace it? we can if we have

newString.length() + A.length() - p > A.length() - k.

 class Solution {
     public String removeKdigits(String num, int k) {
         if (num == null || num.length() ==  || k <= ) return num;
         if (num.length() <= k) return "";
         StringBuilder sb = new StringBuilder();
         for (int p = ; p < num.length(); p++) {
             //这题关键的部分是如果当前这个数比前一个数小，我们就一定要把当前这个数替换掉前一个数。因为这样我们得到的数就一定更小。
             // 但是也不是可以无限替换，我们得保证num后面部分的substring长度+当前sb的长度 >= num.length() - k
             while (sb.length() >=  && num.charAt(p) < sb.charAt(sb.length() - ) && sb.length() > p - k) {
                 sb.deleteCharAt(sb.length() - );
             }
             if (sb.length() < num.length() - k) {
                 sb.append(num.charAt(p));
             }
         }
         // remove the extra 0 at the beginning
         while(sb.length() >  && sb.charAt() == '') {
             sb.deleteCharAt();
         }
         return sb.toString();
     }
 }