leetcode 402 - Remove K Digits

https://leetcode.com/problems/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.

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class Solution {
public:
string removeKdigits(string num, int k) {
stack<char> stk;

for (char n : num) {
while (!stk.empty() && k > 0 && stk.top() > n) {
stk.pop();
k--;
}

stk.push(n);
}

while (k--)
stk.pop();

string result;
while (!stk.empty()) {
result.push_back(stk.top());
stk.pop();
}

reverse(result.begin(), result.end());

int i = 0;
while (i < result.size()) {
if (result[i] != '0')
break;
i++;
}

if (i == result.size())
return "0";
else
return result.substr(i);
}
};