LeetCode 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.

class Solution {
public:
    string removeKdigits(string a, int k) {
	stack s1;
    int n;
	for(int i = 0;a[i] != '\0';i++)
    {
        n = i+1;
    }
    if(n == k)
        return "0";
	int count = k;
	s1.push('0'); //      
	s1.push(a[0]);
	for(int i = 1;i < n;++i)
	{
		while(s1.top() > a[i] && count > 0)
		{
			s1.pop();
			count--;
		}
		s1.push(a[i]);
		
	}
	while(count--)
	{
		s1.pop();
	}
	char c[n-k+1];
	c[n-k] = '\0';
	for(int i = n-k-1;i >= 0;i--)
	{
		c[i] = s1.top();
		s1.pop();
	}
    int i = 0;
    char *p = c; 
    while(c[i++] == '0')
        p = &c[i];
    if(i == n-k+1)
        return "0";
    else
	    return p;
    }
};