https://leetcode.com/problems/remove-k-digits/// 参考了这里的// https://discuss.leetcode.com/topic/59327/o-n-solution/3public class Solution {    public String removeKdigits(String num, int k) {        for (int i=0; i<num.length()-1 && k>0; i++) {            // >=0 必须，因为会减2            if (i>=0 && num.charAt(i) > num.charAt(i+1)) {                num = num.substring(0, i) + num.substring(i+1);                k--;                i-=2;            }        }                // 这一步必须        while (!num.isEmpty() && num.charAt(0) == ‘0‘) {            num = num.substring(1);        }                return num.length() - k >= 1 ? num.substring(0, num.length()-k) : "0";    }}