Remove K Digits https://leetcode.com/problems/remove-k-digits/
- Examples which are interesting: "1234567", "7654321". The first is strictly increasing and the second is strictly decreasing.
- How do you choose to remove the first element?
- You will scan from left to right and remove the first peak element. Example 1 that would be 7, the last element. Example 2 that would be 7, the first
element. Why the first peak element? You want to remove the first largest element from left which has a smaller element on right, so that the smaller element can fill in for the large element that we removed. You will repeat this process k times and have a complexity of O(K * N). https://discuss.leetcode.com/topic/59871/two-algorithms-with-detailed-explaination
- Now how can we do the above efficiently? We can use a stack to simulate this.
- if x > st[-1], push x. If x < st[-1], pop all elements from the stack which are greater than x while making sure we do not pop more than a total of k elements.
- Adjust for boundary conditions.
for x in num: while k and st and x < st[-1]: st.pop() k = k-1 st.append(x)
class Solution(object): def removeKdigits(self, num, k): """ :type num: str :type k: int :rtype: str """ st =  for x in num: while k and st and x < st[-1]: st.pop() k = k-1 st.append(x) while st and k > 0: st.pop() k = k-1 i = 0 while i < len(st) and st[i] == "0": i = i+1 if len(st[i:]) == 0: return "0" else: return "".join(st[i:])