Tags: "leetcode", "stack", "greedy", access_time 2-min read

# Remove K Digits

#### Created: May 14, 2020 by [lek-tin]

##### Last updated: May 14, 2020

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.

### Solution

Java

class Solution {
public String removeKdigits(String num, int k) {

for(char digit : num.toCharArray()) {
while(stack.size() > 0 && k > 0 && stack.peekLast() > digit) {
stack.removeLast();
k -= 1;
}
}

/* remove the remaining digits from the tail. */
for(int i=0; i<k; ++i) {
stack.removeLast();
}

// build the final string, while removing the leading zeros.
StringBuilder res = new StringBuilder();