# Question

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

From left to right, remove digits that are larger than its right one. If less than k of such digits can be found (i.e. the sequence becomes nondecreasing), remove from right until k digits are removed. Remove any leading 0s.

```
from collections import deque
class Solution:
def removeKdigits(self, num, k):
"""
:type num: str
:type k: int
:rtype: str
"""
if k <= 0:
return num
if len(num) == k:
return "0"
stack = deque()
stack.append(num[0])
for c in num[1:]:
while k > 0 and stack and c < stack[-1]:
stack.pop()
k -= 1
stack.append(c)
while k > 0:
stack.pop()
k -= 1
while stack and stack[0] == '0':
stack.popleft()
if not stack:
stack.append('0')
return ''.join(stack)
```