# 402. Remove K Digits

### Problem:

Given a non-negative integer num represented as a string, remove k digits from the number so that the new number is the smallest possible.

Note:

• The length of num is less than 10002 and will be ≥ k.
• The given num does not contain any leading zero. Example 1:
``````Input: num = "1432219", k = 3
Output: "1219"
Explanation: Remove the three digits 4, 3, and 2 to form the new number 1219 which is the smallest.
``````
Example 2:
``````Input: num = "10200", k = 1
Output: "200"
Explanation: Remove the leading 1 and the number is 200. Note that the output must not contain leading zeroes.
``````
Example 3:
``````Input: num = "10", k = 2
Output: "0"
Explanation: Remove all the digits from the number and it is left with nothing which is 0.
``````

### Solutions:

``````public class Solution {
public String removeKdigits(String num, int k) {
if (num.length() == k) {
return "0";
}
StringBuilder sb = new StringBuilder(num);
for (int j = 0; j < k; j ++) {
int i = 0;
while (i < sb.length() - 1 && sb.charAt(i) <= sb.charAt(i + 1)) {
i ++;
}
sb.delete(i, i + 1);
}
int i = 0;
while (i < sb.length() - 1 && sb.charAt(i) == '0') {
i ++;
}
return sb.toString().substring(i);
}
}
``````
``````public class Solution {
public String removeKdigits(String num, int k) {
if (num.length() == k) {
return "0";
}
Stack<Character> stack = new Stack<Character>();
int i = 0;
for (int j = 0; j < k; j ++) {
if (i >= num.length() || (!stack.isEmpty() && stack.peek() > num.charAt(i))) {
stack.pop();
continue;
}
while (i < num.length() - 1 && num.charAt(i) <= num.charAt(i + 1)) {
stack.push(num.charAt(i));
i ++;
}
i ++;
}
while (i < num.length()) {
stack.push(num.charAt(i));
i ++;
}
String sb = "";
while (!stack.isEmpty()) {
sb = stack.pop() + sb;
}
i = 0;
while (i < sb.length() - 1 && sb.charAt(i) == '0') {
i ++;
}
return sb.substring(i);
}
}
``````
``````class Solution {
public String removeKdigits(String num, int k) {
if (num == null || num.length() == 0) {
return "";
}
if (k >= num.length()) {
return "0";
}
for (int i = 0; i < num.length(); i ++) {
char c = num.charAt(i);
while (cands.size() > 0 && c < cands.peekLast() && k > 0) {
cands.pollLast();
k --;
}
}
while (k > 0) {
cands.pollLast();
k --;
}
while (cands.size() > 0 && cands.peekFirst() == '0') {
cands.pollFirst();
}
if (cands.size() == 0) {
return "0";
}
StringBuilder sb = new StringBuilder();
while (cands.size() > 0) {
sb.append(cands.pollFirst());
}
return sb.toString();
}
}
``````