2645. Minimum Additions to Make Valid String
Description
Given a string word
to which you can insert letters "a", "b" or "c" anywhere and any number of times, return the minimum number of letters that must be inserted so that word
becomes valid.
A string is called valid if it can be formed by concatenating the string "abc" several times.
Example 1:
Input: word = "b" Output: 2 Explanation: Insert the letter "a" right before "b", and the letter "c" right next to "b" to obtain the valid string "abc".
Example 2:
Input: word = "aaa" Output: 6 Explanation: Insert letters "b" and "c" next to each "a" to obtain the valid string "abcabcabc".
Example 3:
Input: word = "abc" Output: 0 Explanation: word is already valid. No modifications are needed.
Constraints:
1 <= word.length <= 50
word
consists of letters "a", "b" and "c" only.
Solutions
Solution 1: Greedy + Two Pointers
We define the string $s$ as "abc"
, and use pointers $i$ and $j$ to point to $s$ and $word$ respectively.
If $word[j] \neq s[i]$, we need to insert $s[i]$, and we add $1$ to the answer; otherwise, it means that $word[j]$ can match with $s[i]$, and we move $j$ one step to the right.
Then, we move $i$ one step to the right, i.e., $i = (i + 1) \bmod 3$. We continue the above operations until $j$ reaches the end of the string $word$.
Finally, we check whether the last character of $word$ is 'b'
or 'a'
. If it is, we need to insert 'c'
or 'bc'
, and we add $1$ or $2$ to the answer and return it.
The time complexity is $O(n)$, where $n$ is the length of the string $word$. The space complexity is $O(1)$.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 |
|
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 |
|
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 |
|
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 |
|
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 |
|