Leetcode #2645: Minimum Additions to Make Valid String
In this guide, we solve Leetcode #2645 Minimum Additions to Make Valid String in Python and focus on the core idea that makes the solution efficient.
You will see the intuition, the step-by-step method, and a clean Python implementation you can use in interviews.
Problem Statement
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.
Quick Facts
- Difficulty: Medium
- Premium: No
- Tags: Stack, Greedy, String, Dynamic Programming
Intuition
The problem breaks into overlapping subproblems, so caching results prevents exponential repetition.
A carefully chosen DP state captures exactly what we need to build the final answer.
Approach
Define the DP state and recurrence, then compute states in the correct order.
Optionally compress space once the recurrence is clear.
Steps:
- Choose a DP state definition.
- Write the recurrence and base cases.
- Compute states in the correct order.
Example
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".
Python Solution
class Solution:
def addMinimum(self, word: str) -> int:
s = 'abc'
ans, n = 0, len(word)
i = j = 0
while j < n:
if word[j] != s[i]:
ans += 1
else:
j += 1
i = (i + 1) % 3
if word[-1] != 'c':
ans += 1 if word[-1] == 'b' else 2
return ans
Complexity
The time complexity is , where is the length of the string . The space complexity is .
Edge Cases and Pitfalls
Watch for boundary values, empty inputs, and duplicate values where applicable. If the problem involves ordering or constraints, confirm the invariant is preserved at every step.
Summary
This Python solution focuses on the essential structure of the problem and keeps the implementation interview-friendly while meeting the constraints.