faster than 88.79% of Java online submissions for Remove K Digits.

public class Solution {
    public String removeKdigits(String num, int k) {
        int newLength = num.length() - k;
        char[] stack = new char[num.length()];
        int top = 0;
        for (int i = 0; i < num.length(); i++) {
            char c = num.charAt(i);
            //此处while不能改为if
            //比如：561
            while (k > 0 && top > 0 && stack[top - 1] > c) {
                top--;
                k--;
            }
            stack[top++] = c;
        }

        int offset = 0;
        while (offset < newLength && stack[offset] == '0') {
            offset++;
        }
        //new String(stack, offset, newLength - offset)
        //是为了在递增的情况下进行截取，如1234567
        return offset == newLength ? "0" : new String(stack, offset, newLength - offset);
    }
}