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public class Solution {
    public String removeKdigits(String num, int k) {
    
        int top = 0;
        int len = num.length();
        int left = len - k;
        char[] ans = new char[left];
        for (int i = 0; i < len; i++) {
            char cur = num.charAt(i);
            while (len - i + top > left && top > 0 && cur < ans[top - 1]) {
                top--;
            }
            if (top < left) {
                ans[top++] = cur;
            }
        }
        int j = 0;
        while (j < left && ans[j] == '0') j++;
        return j == left ? "0" : new String(ans, j, left - j);
    }
    
}

public String removeKdigits(String num, int k) {
        if (num == null || num.length() == 0 || k == 0) {
            return num;
        }
        if (k == num.length()) return "0";
        int digits = num.length() - k;
        // stack need to be the same length with num.
        char[] stack = new char[num.length()];
        int top = 0;
        for (int i = 0; i < num.length(); i++) {
            // if the previous character in stack is larger than the current one, then removing it will get a smaller number
            // but we can only do so when k > 0
            while (top > 0 && k > 0 && num.charAt(i) < stack[top - 1]) {
                top--;
                k--;
            }
            stack[top++] = num.charAt(i);
        }
        int j = 0;
        while (j < digits && stack[j] == '0') j++;
        return j == digits ? "0" : new String(stack, j, digits - j);
    }

public class Solution {
    public String removeKdigits(String num, int k) {
        if (num == null || num.length() < 1) return "";
        StringBuilder res = new StringBuilder();
        for (char c : num.toCharArray()) {
            int len = res.length();
            while (len > 0 && c < res.charAt(len - 1) && k > 0) {
                res.setLength(len - 1);
                len--;
                k--;
            }
            res.append(c);
        }
        while (k > 0) {
            int len = res.length();
            res.setLength(len > 1 ? len - 1 : 0); // avoid out of index
            k--;
        }
        //int idx = 0;
        while (res.length() > 0 && res.charAt(0) == '0') {
            res.deleteCharAt(0);
            //idx++;
        }
        return res.length() == 0 ? "0" : res.toString();
    }
}