One pass O(n) Java solution, no stack needed

  • 0
        public String removeKdigits(String num, int k) {
            StringBuilder sb = new StringBuilder(num);
            int count = 0, idx = 0;
            while (count < k && idx < sb.length() - 1) {
                if (sb.charAt(idx) > sb.charAt(idx + 1)) {
                    idx = Math.max(idx - 1, 0);
                } else {
            sb.delete(sb.length() - (k - count), sb.length());
            String res = sb.toString().replaceFirst("^0+", "");
            return res.isEmpty() ? "0" : res;

    The basic idea is to replace the first index i where the ith number is greater than i+1th number. We do this repeatedly until we've hit the end of the string or we have done it k times.

    Then if we haven't replaced for k times, it means that the string is already in ascending order. What we need to do now is removing largest numbers from the string.

    Remember to remove leading 0s if any.

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