Remove K Digits

Desicription

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:

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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:

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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:

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

Solution

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class Solution {
public:
std::string removeKdigits(const std::string& num, int k) {
std::stack<char> mono{};
for(const auto& c : num) {
while(!mono.empty() && mono.top() > c && k > 0) {
mono.pop();
k--;
}
mono.push(c);
}
while(!mono.empty() && k > 0) {
mono.pop();
k--;
}

std::string res = std::string(mono.size(), 'a');
for(int i = mono.size() - 1; i >= 0; i--) {
res[i] = mono.top();
mono.pop();
}
int index = 0;
for(; index < res.size(); index++) {
if(res[index] != '0') {
break;
}
}
return res.empty() ? std::string{"0"} : res.substr(index).empty() ? std::string{"0"} : res.substr(index);
}
};