LintCode Delete 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,
Have you met this question in a real interview? Yes
Example
Given an integer A = "178542", k = 4
return a string "12"

class Solution {
public:
    /**
     *@param A: A positive integer which has N digits, A is a string.
     *@param k: Remove k digits.
     *@return: A string
     */
    string DeleteDigits(string A, int k) {
        // wirte your code here
        string res;
        
        int deleted = 0;
        int len = A.size();
        
        for (int i = 0; i < len; i++) {
            while (!res.empty() && res.back() > A[i] && deleted < k) {
                res.pop_back();
                deleted++;
            }
            if (res.empty() && A[i] == '0') {
                continue;
            }
            res.push_back(A[i]);
        }
        
        return res.substr(0, res.size() - (k - deleted));
    }
};

前导零的删除不计入删除字符数