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,

Example

Given an integer A = "178542", k = 4

return a string "12"

Runtime: 13ms.

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
        
        // find the first decrease index and erase index - 1
        // continue with the new string
        if (k == A.size()) return "";
        
        int count = 0;
        while (count++ < k) {
            bool erased = false;
            for (int i = 1; i < A.size(); i++) {
                if (A[i] < A[i - 1]) {
                    A.erase(i - 1, 1);
                    erased = true;
                    break;
                }
            }
            if (!erased) A.erase(A.size() - 1, 1);
        }
        // ensure that there is no leading 0s in A
        int i = 0;
        while (i < A.size() && A[i] == '0') i++;
        return A.substr(i, A.size() - i);
    }
};