Remove K Digits

https://www.lintcode.com/problem/remove-k-digits/description

public class Solution {
    /**
     * @param num: a string
     * @param k: an integer
     * @return: return a string
     */
    public String removeKdigits(String num, int k) {
        // write your code here
        if (num.length() <= k) {
            return "0";
        }
        char[] array = num.toCharArray();
        StringBuilder stringBuilder = new StringBuilder();
        int startIndex = 0;
//        最后的位数应该是
        int finalLength = num.length() - k;
        while (stringBuilder.length() < finalLength) {
//            已经有的位数是
            int length = stringBuilder.length();
            int remianlength = finalLength - length;
//            右边界是
            int right = num.length() - remianlength;
            char temp = array[startIndex];
            startIndex++;
            for (int i = startIndex; i <= right; i++) {
                if (array[i] < temp) {
                    temp = array[i];
                    startIndex = i + 1;
                }
            }
            stringBuilder.append(temp);
        }
        String s = stringBuilder.toString();
        while (s.length() > 1 && s.startsWith("0")) {
            s = s.substring(1);
        }
        return s;
    }
}