402. Remove K Digits

Given a non-negative integer num represented as a string, remove k digits from the number so that the new number is the smallest possible.

Note:

• The length of num is less than 10002 and will be ≥ k.

• The given num does not contain any leading zero.

Example 1:

Input: num = "1432219", k = 3
Output: "1219"
Explanation: Remove the three digits 4, 3, and 2 to form the new number 1219 which is the smallest.

Example 2:

Input: num = "10200", k = 1
Output: "200"
Explanation: Remove the leading 1 and the number is 200. Note that the output must not contain leading zeroes.

Example 3:

Input: num = "10", k = 2
Output: "0"
Explanation: Remove all the digits from the number and it is left with nothing which is 0.

string removeKdigits(string num, int k) { // time: O(n); space: O(n)
    string res;
    int n = num.length(), keep = n - k;
    for (char c : num) {
        while (k && !res.empty() && res.back() > c) {
            res.pop_back();
            --k;
        }
        res.push_back(c);
    }
    res.resize(keep);
    while (!res.empty() && res[0] == '0') res.erase(res.begin());
    return res.empty() ? "0" : res;
}