【LeetCode】402. 移掉K位数字

class Solution {
public:
    string removeKdigits(string num, int k) {
        vector<char> stk;
        for (auto& digit: num) {
            while (stk.size() > 0 && stk.back() > digit && k) {
                stk.pop_back();
                k -= 1;
            }stk.push_back(digit);
        }
        for (; k > 0; --k) stk.pop_back();
        string ans = "";
        bool isLeadingZero = true;
        for (auto& digit: stk) {
            if (isLeadingZero && digit == '0') continue;
            isLeadingZero = false;
            ans += digit;
        }
        return ans == "" ? "0" : ans;
    }
};