402. Remove K Digits

class Solution {
    public String removeKdigits(String num, int k) {
        // greedy with stack
        Deque<Character> stack = new LinkedList<>();
        
        for(int i=0; i<num.length(); i++){
            if(k>0 && stack.size()>0 && num.charAt(i)<stack.peek()){
                stack.pop();
                k--;
                i--;
                continue;
            }
            stack.push(num.charAt(i));
        }
        while(k>0){
            stack.pop();
            k--;
        }
        
        StringBuilder sb = new StringBuilder();
        while(stack.size()>0){
            // deque method: last is the bottom element of the stack
            if(stack.getLast()=='0' && sb.length()==0){
                stack.removeLast();
                continue;
            }else{
                // deque method: removeLast()
                sb.append(stack.removeLast());
            }
        }
        return sb.length()>0? sb.toString() : "0";
    }
}