[LeetCode] Problem 763 - Partition Labels

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A string S of lowercase letters is given. We want to partition this string into as many parts as possible so that each letter appears in at most one part, and return a list of integers representing the size of these parts.

Example

Input: S = “ababcbacadefegdehijhklij”

Output: [9,7,8]

Explanation:
The partition is “ababcbaca”, “defegde”, “hijhklij”.
This is a partition so that each letter appears in at most one part.
A partition like “ababcbacadefegde”, “hijhklij” is incorrect, because it splits S into less parts.

Note

S will have length in range [1, 500].
S will consist of lowercase letters (‘a’ to ‘z’) only.

Code

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public List<Integer> partitionLabels(String S) {
    List<Integer> result = new ArrayList<>();
    int[] last = new int[26];

    for (int i = 0; i < S.length(); i++) {
        int idx = S.charAt(i) - 'a';
        last[idx] = i;
    }

    int start = 0;
    int end = 0;

    for (int i = 0; i < S.length(); i++) {
        int idx = S.charAt(i) - 'a';
        end = Math.max(end, last[idx]);

        if (i == end) {
            result.add(end - start + 1);
            start = i + 1;
        }
    }

    return result;
}