402. Remove K Digits

class Solution {
    public String removeKdigits(String num, int k) {
        StringBuilder sb = new StringBuilder(); // 非严格单调递增

        for (int i = 0; i < num.length(); i++) {
            char c = num.charAt(i);
            while (k > 0 && sb.length() > 0 && c < sb.charAt(sb.length() - 1)) {
                sb.deleteCharAt(sb.length() - 1);
                k--;
            }

            sb.append(c);
        }

        // 注意 k 必须用完
        while (k > 0) {
            sb.deleteCharAt(sb.length() - 1);
            k--;
        }

        String res = sb.toString();
        int firstNonZeroIndex = 0;
        while (firstNonZeroIndex < res.length() && res.charAt(firstNonZeroIndex) == '0') {
            firstNonZeroIndex++;
        }
        if (firstNonZeroIndex == res.length()) {
            return "0";
        } else {
            return res.substring(firstNonZeroIndex);
        }
    }
}

References

402. Remove K Digits