Partition Labels

A string S of lowercase English 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 1:

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 English letters ('a' to 'z') only.

class Solution {
    public List<Integer> partitionLabels(String S) {
        if (S == null || S.length() == 0) {
            return null;
        }
        List<Integer> list = new ArrayList<>();
        int[] map = new int[26]; // record the last index of the each char

        for (int i = 0; i < S.length(); i++) {
            map[S.charAt(i) - 'a'] = i;
        }
        // record the end index of the current sub string
        int last = 0;
        int start = 0;
        for (int i = 0; i < S.length(); i++) {
            // last shows the biggest last inde for our current substring
            // now if we get i==last that means all the chars in this substring have all 
            // repetitions between start and last
            last = Math.max(last, map[S.charAt(i) - 'a']);
            if (last == i) {
                list.add(last - start + 1);
                start = last + 1;
            }
        }
        return list;
    }
}