# Remove K Digits Problem

## Description

LeetCode Problem 402.

Given string num representing a non-negative integer num, and an integer k, return the smallest possible integer after removing k digits from num.

Example 1:

``````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:

``````1
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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:

``````1
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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.
``````

Constraints:

• 1 <= k <= num.length <= 10^5
• num consists of only digits.
• num does not have any leading zeros except for the zero itself.

## Sample C++ Code

``````1
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class Solution {
public:
string removeKdigits(string num, int k) {
// treat ans as a stack in below for loop
string ans = "";

for (char c : num) {
while (ans.length() && ans.back() > c && k) {
// make sure digits in ans are in ascending order
ans.pop_back();
// remove one char
k--;
}
if (ans.length() || c != '0')
ans.push_back(c);
}

while (ans.length() && k--)
// make sure remove k digits in total
ans.pop_back();

return ans.empty() ? "0" : ans;
}
};
``````