RemoveKDigits.java

class Solution {
    public String removeKdigits(String num, int k) 
    {
        if(k == num.length())
            return "0";
        
        Stack<Character> stack = new Stack<>();
        
        int i=0;
        
        while(i < num.length())
        {
            char ch = num.charAt(i);
            //whenever meet a digit which is less than the previous digit, discard the previous one
            while(!stack.isEmpty() && stack.peek() > ch && k > 0)
            {
                stack.pop();
                k--;
            }
            stack.push(ch);
            i++;
        }
        //for corne case like 1111
        while(k > 0)
        {
            stack.pop();
            k--;
        }
        
        //constructing number from stack
        
        StringBuilder sb = new StringBuilder();
        
        while(!stack.isEmpty())
            sb.append(stack.pop());
        
        sb.reverse();
        
        while(sb.charAt(0) == '0' && sb.length() > 1)
            sb.deleteCharAt(0);
        
        return sb.toString();
    }
}