Optimal Partition of String

Published: Sep 21, 2022

Introduction

To solve string partition problems, hash table or set is used to save a previous occurrence. If the problem asks a length, it needs hash table. However, this problem asks just a number of groups, set is a good data structure to solve.

Problem Description

Given a string s, partition the string into one or more substrings such that the characters in each substring are unique. That is, no letter appears in a single substring more than once.

Return the minimum number of substrings in such a partition.

Note that each character should belong to exactly one substring in a partition.

Constraints:

• 1 <= s.length <= 10**5
• s consists of only English lowercase letters.

https://leetcode.com/problems/optimal-partition-of-string/

Examples

Example 1
Input: s = "abacaba"
Output: 4
Explanation:
Two possible partitions are ("a","ba","cab","a"), ("ab","a","ca","ba"), and ("ab","ac","ab","a").

Exammle 2
Input: s = "ssssss"
Output: 6
Explanation:
The only valid partition is ("s","s","s","s","s","s").

Analysis

Use a set to save the previous occurrence of a same character. If the same character appears, count up and clear the set. In the end, if the set is not empty, count up.

Solution

class OptimalPartitionString:
    def partitionString(self, s: str) -> int:
        count, seen = 0, set()
        for c in s:
            if c in seen:
                count += 1
                seen.clear()
            seen.add(c)
        return count + 1 if seen else count

Complexities

• Time: O(n)
• Space: O(n)