Sliding Window || C++ Solution
IntuitionApproachComplexity
Time complexity:

Space complexity:
Code
# Intuition
<!-- Describe your first thoughts on how to solve this problem. -->

# Approach
<!-- Describe your approach to solving the problem. -->

# Complexity
- Time complexity:
<!-- Add your time complexity here, e.g. $$O(n)$$ -->

- Space complexity:
<!-- Add your space complexity here, e.g. $$O(n)$$ -->

# Code
```cpp []
class Solution {
public:
    string lexicographicallySmaller(const string& a, const string& b) {
        return (a < b) ? a : b;
    }

    string shortestBeautifulSubstring(string s, int k) {
        string ans = "";
        int cnt = 0;  
        int j = 0;    

        for (int i = 0; i < s.length(); i++) {
            if (s[i] == '1') cnt++;  

            while (cnt == k && j <= i) {
                if (ans.empty() || ans.size() > (i - j + 1)) {
                    ans = s.substr(j, i - j + 1);
                } 
                
                else if (ans.size() == (i - j + 1)) {
                    ans = lexicographicallySmaller(ans, s.substr(j, i - j + 1));
                }

                if (s[j] == '1') cnt--;  
                j++;  
            }
        }

        return ans;
    }
};
```
['C++']